After a hepadnavirus inserts itself into a chromosome of an animal, fragments of the virus are passed on to all of that animal' s descendants. A hepadnavirus fragment is present in a chromosome of the zebra finch and in precisely the same location in a corresponding chromosome of the dark-eyed junco. The fact that these two bird species diverged from each other about 25 million years ago ttherefore means that the hepadnavirus is at least 25 million years old. Which one of the following, if true, most strengthens the argument? ["When a virus inserts itself into an animal's chromosome, the insertion occurs at a random spot.", "The presence of a hepadnavirus in an animal species does not affect the likelihood of that species' survival.", 'The chromosomes of the zebra finch and the dark-eyed junco contain fragments of no virus other than the hepadnavirus.', 'Viruses can affect the evolution of an organism and can thereby influence the likelihood of their diverging into two species.']