The correct answer is (C).
(C) Strengthen
Step 1: Identify the Question Type
The question directly asks for something that strengthens the argument.
Step 2: Untangle the Stimulus
The author concludes (therefore) that the hepadnavirus is at least 25 million years old. The evidence is that the hepadnavirus inserts itself into an animal's chromosome and then gets passed down from generation to generation. One hepadnavirus fragment is found in chromosomes of both zebra finches and juncos, bird species that diverged from each other 25 million years ago. In addition, the virus fragment is found in the exact same location in corresponding chromosomes.
Step 3: Make a Prediction
The author is suggesting that the virus inserted itself into a bird at least 25 million years ago. It was genetically passed down until 25 million years ago, when that bird species diverged into two separate birds: zebra finches and juncos. The strongest evidence is the placement of the virus. Both birds have the virus in the exact same location in a similar chromosome. It can't be a coincidence . . . or could it? Maybe the virus always inserts itself in the same location of a chromosome. If that were true, the virus could have inserted itself at two different times, perhaps just 1,000 years ago. The author assumes otherwise: the virus inserted itself once at least 25 million years ago, and the identical placement is not a coincidence. The correct answer will verify this assumption.
Step 4: Evaluate the Answer Choices
C) is correct, validating the assumption that the virus inserted itself just once. If the placement of the virus is always random, it would be too coincidental that the virus was found in the exact same spot in both bird's chromosomes. It makes it much more likely that it was inserted once over 25 million years ago and got passed along during the split.
(A) is a Distortion. Even if viruses did have this ability, there's no evidence that the split wasn't caused by a different virus and the hepadnavirus inserted itself at different, much more recent times.
(B) is irrelevant. Even if no other viruses are present, there's still no evidence that the hepadnavirus fragments are genetically linked to the same virus from before the species diverged.
(D) is Out of Scope. There would still be the question of whether those fragments appear in the very same spot in those birds' chromosomes. If they did, then the author's argument seems less valid because if unrelated bird species all have the virus show up in the same spot of the chromosome, then the placement of the virus is not random, and thus, the zebra finch and dark-eyed junco need not have both had the virus at the time of their split. They could have each contracted the virus at different times much more recently than 25 million years ago.
(E) is also Out of Scope. Even if the hepadnavirus doesn't affect a species' survival, there's still no evidence that the same virus inserted itself into a bird 25 million years ago.
