sat blue book grammar question?

catepillargiraf
Mon, 06 Jul 2015 22:42:58 +0000

(Until being widely hunted for its) ivory and blubber in the eighteenth century, walruses were plentiful in the waters of the northeaster United States.

A. Until being widely hunted for its
B. Before having been widely hunted for its
C. Up to them being widely hunted for their
D. Until they were widely hunted for their
E. Before they have been widely hunted for their

I get that the answer is D, but can anyone explain why the rest is not right?

I particularly don’t understand why B and E is wrong, although it sounds wrong. What’s the difference between having been and have been, and what is the difference between had been and have been?