Ringtail opossums are an Australian wildlife species that is potential

When we need to support (i.e. strengthen) an argument, we look for a choice that makes us believe the author's conclusion.

While there are many specific ways to strengthen arguments, almost all strengtheners fall into one of two general categories:

1) give you another reason to believe the conclusion; or
2) remove a reason NOT to believe the conclusion.

Many test takers have more trouble identifying the second category than the first.

Let's look at this particular argument:

C: native opossums are endangered by non-native predators

E: 75% of group raised in captivity were killed by non-native predators

The great thing about the GMAT is that it's a standardized test - the same patterns show up over and over and over. That fact is as true in verbal as it is in math.

In CR, for example, certain classic argument structures run rampant. One of the most common argument structures is representativeness.

In a representativeness argument, information gathered from a study/survey/poll/sample or experiment is used to draw a conclusion. In every such argument, the author assumes that the group in the evidence is representative of the group in the conclusion.

This argument is a prime example - we have evidence about a group of opossums raised in captivity and a conclusion about all opossums. For the conclusion to be valid, the author must be assuming that opossums raised in captivity are representative of all opossums.

Since this is a strengthening question, we predict: the correct answer will make us think that the two groups are substantially similar in all relevant respects.

Now let's look at the choices:

(A) There are fewer non-native predator species that prey on the ringtail opossum than there are native species that prey on the ringtail opossum.

Nothing about the 2 groups - eliminate.

(B) Foxes, which were introduced into Australia over 200 years ago, adapted to the Australian climate less successfully than did some other foreign species.

Nothing about the 2 groups - eliminate.

(C) The ringtail opossums that were raised in captivity were fed a diet similar to that which ringtail opossums typically eat in the wild.

Comparison of the 2 groups - worthy of our attention. However, are the diets a relevant similarity? NO - we don't know the connection between diet and susceptibility to non-native predators - eliminate.

(D) Few of the species that compete with the ringtail opossum for food sources are native to Australia.

Nothing about the 2 groups - eliminate.

(E) Ringtail opossums that grow to adulthood in the wild defend themselves against foxes no more successfully than do ringtail opossums raised in captivity.

Comparison of the 2 groups - worthy of our attention. Is defense against foxes relevant? YES! (E) eliminates the possibility that wild opossum are better at self-defense than previously captive opossum, which would weaken the argument. Since (E) eliminate a weakener, it strengthens the conclusion. Choose (E)!