Answer: C. Their continuously-growing teeth.
Rodents’ teeth are specialized for gnawing; in fact, the word “rodent” has its origin in the Latin word rodere, meaning “to gnaw.” They have two pairs of incisors, one on the upper jaw and one on the lower; these teeth grow continuously throughout their life, and must be kept short and sharp through gnawing.
(Of course, there are exceptions to every rule: there is one species of earthworm-eating shrew-rat that does not gnaw, and has only two teeth in total. Rabbits and other lagomorphs also have continuously growing teeth, but unlike rodents, they have four incisors in the upper jaw, not two.)
Question #2: What percent of all mammal species are rodents?