求助，一道OG语法题

The golden crab of the Gulf of Mexico has not been fished commercially in great numbers, primarily on account of living at great depths-- 2,500 to 3,000 feet down.

(A)  on account of living

(B)  on account of their living

(C)  because it lives

(D)  because of living

(E)   being they live

As used in choices A, B, and D, the phrases on account
   of and because of are unidiomatic; because, which appears in C and E, is preferable here since because can introduce a complete subordinate clause explaining the reason why the golden crab has not been fished extensively.
   B and E also produce agree­ment errors by using the plural pronouns their and they to refer to the singular noun crab. Choice D, like A, fails to provide a noun or pronoun to perform the action of living, but even with its the phrases would be more awkward and less clear than it lives. C, which uses because and it as the singular subject of a clause, is the best choice

The+n。不是表示泛指一类吗？后面用复数V.，这里的意思也应该是指一类，为什么后面却用单数V.？