请教03-01的2道语法题

Newfish

12. Before the Europeans arrived, American Indians were using virgin copper …... into ornaments, knives, and other artifacts.

(A) Which was hammering

(B) which hammered

(C) was hammered

(D) hammered

答案是D，B和D有什么区别吗，为什么不可以选B呢？

24. Lightning tends to strike the nearest good conductor, and hence often strikes

A                       B

in same place more than once.

答案是C，如何改啊。

谢谢