Q:Information:  - Rodents (from Latin "rodere", "to gnaw") are mammals of the order Rodentia, which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents; they are found in vast numbers on all continents except Antarctica. They are the most diversified mammalian order and live in a variety of terrestrial habitats, including human-made environments.  - Agnotocastor is an extinct member of the beaver family Castoridae . Unlike its modern relative , this species took the place of muskrats in the rivers of North America during the Oligocene epoch .  - The family Castoridae contains the two living species of beavers and their fossil relatives. This was once a highly diverse group of rodents, but is now restricted to a single genus, "Castor".  - The beaver (genus Castor) is a large, primarily nocturnal, semiaquatic rodent. "Castor" includes two extant species, the North American beaver ("Castor canadensis") (native to North America) and Eurasian beaver ("Castor fiber") (Eurasia). Beavers are known for building dams, canals, and lodges (homes). They are the second-largest rodent in the world (after the capybara). Their colonies create one or more dams to provide still, deep water to protect against predators, and to float food and building material. The North American beaver population was once more than 60 million, but as of 1988 was 612 million. This population decline is the result of extensive hunting for fur, for glands used as medicine and perfume, and because the beavers' harvesting of trees and flooding of waterways may interfere with other land uses.    After reading the paragraphs above, we are interested in knowing the entity with which 'agnotocastor' exhibits the relationship of 'taxon rank'. Find the answer from the choices below.  Choices: - family  - genus  - group  - order  - species  - variety
A:
genus