Summary
At night, a flock of crows will usually perch in a small place called a roost.
Every morning, the crows leave the roost to hunt and scavenge. Most flocks of crows keep their hunting to within 100 to 130 km from the roost.
Normally a flock of crows stays in the same roost for several years.
When a flock abandons a roost to form a new roost, the new roost is usually less than 8 km away from the abandoned roost.
Every morning, the crows leave the roost to hunt and scavenge. Most flocks of crows keep their hunting to within 100 to 130 km from the roost.
Normally a flock of crows stays in the same roost for several years.
When a flock abandons a roost to form a new roost, the new roost is usually less than 8 km away from the abandoned roost.
Notable Valid Inferences
There’s no clear inference from the stimulus. I’d rely on process of elimination to identify which answer can most justifiably be rejected.
A
Crows will abandon their roost site only in response to increases in the population of the flock.
No reason to reject. We have no reason to think population changes aren’t requires in order for crows to abandon their roost.
B
When there is a shortage of food in the area in which a flock of crows normally hunts and scavenges, some members of the flock will begin to hunt and scavenge outside that area.
No reason to reject. Although we know crows typically stay within a certain hunting range, that doesn’t suggest some crows won’t go outside that range.
C
Most of the hunting and scavenging that crows do occurs more than eight kilometers (five miles) from their roost.
No reason to reject. We know the general range of hunting, but we don’t know anything about where, within that range, crows do most of their hunting and scavenging.
D
Once a flock of crows has settled on a new roost site, it is extremely difficult to force it to abandon that site for another.
No reason to reject. We don’t know anything about the difficulty of getting a flock to abandon a roost.
E
When a flock of crows moves to a new roost site, it generally does so because the area in which it has hunted and scavenged has been depleted of food sources.
Have reason to reject. When crows move to a new roost, the new one is usually less than 8 km away from the old roost. That means the hunting ranges of the old and new roosts largely overlap. That suggests the crows aren’t motivated by looking for new food sources when they move.
Summarize Argument
The sociologist concludes that people who want to be happy shouldn’t own pets. This is because pet-owners are generally less happy than people who don’t have pets.
Notable Assumptions
The sociologist assumes people who own pets weren’t less happy before owning one. This means that she assumes pets don’t positively contribute to happiness, perhaps by providing companionship to otherwise lonely people who are likely to be unhappy.
A
Some people who have pets are happier than most people who do not.
“Some” simply means “more than one.” This doesn’t weaken a general argument about pet ownership.
B
Most people who have no pets occasionally wish that they had pets.
It doesn’t matter what people sometimes wish. We have data saying that pet-owners are, all else equal, less happy that people who don’t own pets.
C
Most people who have pets are reasonably happy.
Even if they’re reasonably happy, they’re not as happy as people who don’t have pets. The conclusion concerns people who want to be “as happy as possible.”
D
Most people who have pets feel happier because they have pets.
Pets generally make people happier. So, people who want to be as happy as possible might actually do well to get a pet. This weakens the sociologist’s argument.
E
All people who have no pets admit to feeling unhappy sometimes.
Even if people who don’t own pets are unhappy sometimes, they’re still happier overall than people who own pets.
A
When webs of many different species of spider were illuminated with a uniform source of white light containing an ultraviolet component, many of these webs did not reflect the ultraviolet light.
Irrelevant. The author never said that Glomosus spiderwebs are the only spiderwebs that reflect ultraviolet light.
B
When the silks of spiders that spin silk only for lining burrows and covering eggs were illuminated with white light containing an ultraviolet component, the silks of these spiders reflected ultraviolet light.
Like (A), this is irrelevant. We don’t care about other spiderwebs that reflect ultraviolet light.
C
When webs of the comparatively recently evolved common garden spider were illuminated with white light containing an ultraviolet component, only certain portions of these webs reflected ultraviolet light.
Irrelevant. We don’t care about other spiderwebs that reflect ultraviolet light.
D
When Drosophila fruit flies were placed before a Glomosus web and a synthetic web of similar pattern that also reflected ultraviolet light and both webs were illuminated with white light containing an ultraviolet component, many of the fruit flies flew to the Glomosus web.
The author argues that the specific patterns of ultraviolet light attract insects. This suggests something else about the Glomosus spiderwebs is actually what attracts them.
E
When Drosophila fruit flies were placed before two Glomosus webs, one illuminated with white light containing an ultraviolet component and one illuminated with white light without an ultraviolet component, the majority flew to the ultraviolet reflecting web.
Between the otherwise identical Glomosus spiderwebs, insects chose the one with ultraviolet reflections. This strengthens the idea that ultraviolet reflections, and not something else about the spiderwebs, are what attract insects.