Self-study
Support All zebras have stripes, and the most widespread subspecies has the best-defined stripes. ███ ███████ ████ █████████ ██ ██ ██████████ ██ ███ ████████ █████ █████ █████ █████████ ███████ ███ ███████ ███ ██████ ████████ ██ ███████████ ████ ████ █████ ██ ████ ████ ██ ██████ ███ █████ ███████
Argument Breakdown
The author concludes that zebras' stripes must serve a function of signaling to other zebras. In support, the author explains that all zebras have stripes, with the most common zebra subspecies having the best-defined stripes. This leads to a sub-conclusion that stripes serve some important function for zebras. As a final piece of support, the author states that zebras' stripes cannot function as camouflage.
The author is making a phenomenon-hypothesis argument, where the conclusion is a hypothesis advanced to explain the observed phenomenon of zebras' stripes. The author's premises define limits on possible explanations: given the premises, the hypothesis has to state that stripes serve some important function, which is not camouflage. According to the author, this function is social signaling.
Objective: Strengthen
There are several possible approaches to strengthening a phenomenon-hypothesis argument. The correct answer could rule out an alternative explanation, for example by saying that stripes don't serve a physiological function like temperature regulation. The correct answer could also offer corroborating evidence, such as a specific example of stripes serving a signaling function. Another possibility is presenting a causal mechanism for how this signaling functions.
Whatever the exact approach the correct answer takes, it will add additional support to the conclusion by making the link between the premises and conclusion stronger. Pay attention to the exact details of each answer choice: an answer will only be correct if it gives us a specific reason to believe in the conclusion that zebras' stripes perform a signaling function.
7.
Which one of the following, ██ █████ ████ ████████ ████████ ███ ██████████ █████████ █ █████████ █████████
a
The subspecies of ██████ ████ ███ ████████████ ███████ ██ ████ █████████████ ██ ███████████ ████ ███ ██████
(A) might look like it raises additional benefits for stripes, but there are two problems that prevent (A) from strengthening. First, size and vigor are great, but they're unrelated to signaling. These benefits don't actually relate to the author's hypothesis.
Second, as far as we know these advantages are unrelated to the zebras' stripes—we have no reason to think that well-defined stripes cause exceptional size and vigor. In fact, this undermines the author's support about the importance of stripes by giving another reason why the most common subspecies is successful.
b
In certain tall ███████ ██████ ███ ██ ██████ ██ ████ ████ ███████ ███████ ████ █ ████ ██ ███████ ██████
(B) seems to contradict a premise by indicating that stripes can be camouflaging in certain conditions. This doesn't help us strengthen, because by opening the door to a camouflage explanation, it removes support from the conclusion that stripes serve a signaling function.
c
A visual signal ███████████ █████ ███ ███████ ██ █ ███████ ███ ███████ ██ █ █████████ ██████ ██ █████ ███████████ ██ █████ ███████ ██ ███ ████████
(C) tells us about one possible mechanism for visual signaling, but as far as we know, it's not applicable to zebras. This is because we have no reason to believe that zebras change color, or if they did, that such a color change would be related to their stripes in any way.
Because (C) isn't specific to zebras, it can't support the author's specific hypothesis about signaling in zebras. All (C) gives us is general information about how signaling might work in some species. Without telling us if zebras are even one of those species, (C) can't help.
d
Zebras react much ██████ ██ ██████ ██████ ████ ████ ███████ ████ ████ ██ ██ ██████ ██████ ████ ███ █████████ █████████ ███ ████ ████████
(D) strengthens by giving us corroborating evidence that aligns with the author's hypothesis. If the hypothesis were true and stripes were used for signaling, we would expect zebras to react in a distinct way to something that looks like a striped animal. (D) tells us that what we would expect given the hypothesis is true, which gives us more reason to believe the author's conclusion.
e
Zebras have a ██████ ██████████ ██ █████ ███████ ████ ██ ███████ ███████ ████ ██ ███████
(E) discusses signals, but vocal signals rather than visual signals like stripes. Because (E) has no relevance to visual signaling, it can't strengthen the author's case that this is the function of zebras' stripes. If anything, saying that zebras have a wider range of vocal signaling options would make stripes less useful for signaling.