Deep tillage is even more deleterious to the world’s topsoil supply than previously believed. For example, farmers who till deeply are ten times more likely to lose topsoil to erosion than are farmers who use no-till methods. Results like these make it clear that farmers who now till deeply should strive, by using other topsoil aeration techniques, to incorporate no-till methods instead.

Summary
The author concludes that farmers who now deep-till should try to use no-till methods. Why? Because farmers who till deeply are much more likely to lose topsoil to erosion than are farmers who use no-till methods.

Notable Assumptions
The author concludes that farmers using deep-till methods should switch to no-till methods...but aren’t we overlooking other potential options? What if a shallow-till method is less likely to damage topsoil than both deep-till and no-till? The author assumes that there’s no other method that would be more effective at reducing topsoil loss than deep-till and no-till.

A
Topsoil erosion does not make farmers want to till more deeply.
What farmers “want” to do is irrelevant to the argument. What matters is what farmers actually do. Farmers may “want” to till more deeply in response to topsoil, but that doesn’t undermine the argument because it doesn’t imply that farmers in fact are tilling more deeply in response to topsoil erosion.
B
In deep-tillage farming, the deeper one tills, the greater the susceptibility to topsoil erosion.
Not necessary, because the author doesn’t have to assume the “deeper” the till, the “greater” the susceptibility. It’s possible, for example, that deep-tilling leads to more topsoil erosion than shallow tilling, but that effect happens only after we exceed a certain depth.
C
Tilling by any method other than deep tillage is not a viable option.
Necessary, because if this weren’t true — if tilling by another method besides deep tillage IS a viable option — then the author has not proven that current deep-till farmers should switch to no-till. Maybe they should switch to some other till method besides deep-till? The author must assume that these other options aren’t viable.
D
The most expensive farming methods employ topsoil aeration techniques other than deep tillage.
The “most expensive farming methods” have no connection to the reasoning of this argument.
E
On average, topsoil that is no-tilled is more aerated than topsoil that is tilled deeply.
Not necessary, because we don’t know the relationship between greater or less aeration and amount of topsoil loss to erosion. If, on average, topsoil that is no-tilled is NOT more aerated than topsoil that is tilled deeply, that doesn’t undermine the argument, because we’d still have evidence that the deep-tillers are more likely to experience topsoil loss.

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Two doctrines have been greatly influential in this century. The first holds that the explanation of any historical event must appeal to economic factors. The second attempts to account psychologically for all historical events, especially in terms of early childhood experience. Both doctrines, however, are mistaken. Certainly there have been events that were due both to economic forces and to the nature of the early childhood experiences of the major participants in the event.

Summary
Doctrine 1 says that explanations of any historical event must appeal to economic factors.
Doctrine 2 tries to explain all historical events in psychological terms.
The author concludes that both doctrines are wrong. Why? Because there are events that were due both to economics and psychological forces.

Notable Assumptions
Why does the author think that the fact some events need both economic and psychological explanations shows that the doctrines are wrong? The author’s line of reasoning doesn’t make sense, because neither of the doctrines say that events are exclusively caused by economics or exclusively caused by psychology. This gets to the core of the author’s assumption. He’s assuming that Doctrine 1 believes events are exclusively explained by economics, and that Doctrine 2 believes events are exclusively explained by psychology.

A
The first doctrine precludes any noneconomic factors in explanations of historical events.
(A) is necessary, because if it were not true — if Doctrine 1 allows noneconomic factors in explanations of historical events — then the author’s premise wouldn’t show that Doctrine 1 is wrong. The author mistakenly believes that Doctrine 1 doesn’t allow for noneconomic factors; this is why he thinks pointing out some events involve both economic and non-economic factors goes against Doctrine 1.
B
The second doctrine places importance only on childhood experiences.
The author doesn’t have to believe Doctrine 2 places importance only on childhood experiences. Doctrine 2 may focus on early childhood experiences, but there’s no indication the author thinks no other psychological experiences are relevant to Doctrine 2.
C
Historical events are influenced as much or as little by economic factors as by psychological factors.
The author simply points out that some events involve both factors. But one kind of factor can be more influential than the other; that wouldn’t undermine the author’s line of reasoning. To the author, as long as an event involves both factors — regardless of whether they are equally influential or not — that shows both doctrines are wrong.
D
One is likely to find that both economic and psychological explanations have been proposed for any given historical event.
The author simply points out that there are some events that involve both factors. But the author doesn’t assume that both kinds of explanations are likely to have been proposed for “any” historical event.
E
Appeals to both economic and psychological factors are needed to understand any historical event properly.
The author simply points out that there are some events that involve both factors. But the author doesn’t assume that both factors are necessary to understand “any” historical event.

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Paleontologist: It is widely, but falsely, held that life began in the ocean and did not exist on land until half a billion years ago. Traces of carbon 14 have been found throughout certain 1.2-billion-year-old rocks in North America. Carbon 14 is extracted by plants and microbes from the atmosphere to fuel their chemical reactions, and is then released when the plants and microbes die.

Summarize Argument: Phenomenon-Hypothesis
The author hypothesizes that life existed on land earlier than half a billion years ago or that life began on land. This is based on the fact that traces of carbon 14 have been found throughout certain 1.2-billion-year old rocks. Carbon 14 is extracted from the atmosphere by living things and is then released when those living things die.

Notable Assumptions
The author assumes that there’s no other explanation for the presence of carbon 14 in the 1.2-billion-year-old rocks besides the existence of life. This overlooks the possiiblity that carbon 14 might get into the rock in ways that don’t require life to have existed.

A
According to one dating technique, a few fossils of plants that lived on land are more than half a billion years old.
This provides additional evidence that life existed earlier than half a billion years ago.
B
The severity of conditions in the primordial oceans would have made it difficult for life to begin there.
This provides a reason to think life did not begin in the ocean.
C
Research suggests that some portions of the 1.2-billion-year-old rocks were at one time submerged in water, though other portions clearly never were.
This strengthens by establishing that some portions of the rocks were never under water. This helps to eliminate the possiblity that all of the carbon 14 in the rocks originated from undersea creatures.
D
The 1.2-billion-year-old rocks were formed from soil containing very small amounts of carbon 14 that entered the soil directly from the atmosphere.
This weakens the argument by providing an alternate source of the carbon 14 besides life. (D) suggests the carbon 14 in the rocks came directly from the atmosphere rather than from a living creature.
E
Uranium testing has been used to confirm that the age of the rocks is 1.2 billion years.
This strengthens the argument by confirming that the rocks originate from earlier than half a billion years ago. If this were not true, the argument would be less convincing.

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