I would try not to think too much about scope. We need to ask ourselves how each answer choice relates to what we just read.
If the virus did not insert itself in a random spot, what would that mean? Perhaps there are only a few spots that it could inset itself. If this were true, the argument would be much weaker. Perhaps the similarity of the spots in mere coincidence.
C seals off this possibility, if the assignment is random, then it is exceedingly unlikely that the similarity of the virus' placement is a coincidence, which makes it more likely that the virus impacted a common ancestor.
This strengthens the argument by weakening a potential rebuttal.
