48. 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
 
1．代词指代, 数的一致：（一级排除B,E）

2. 词法 :  (because句子优选) ：（二级排除A,D）
A. because + 完整的句子
B. on account of / because of + n. ==》 此处没有明确的限定名词的内涵
C. Because 》on account of / because of
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
. C, which uses because and it as the singular subject of a clause, is the best choice.

(言下之意，主谓宾说的最清楚)

3.一般现在式表示持续发生的事实和真理。