og 39

39. For members of the seventeenth-century Ashanti nation in Africa, animal-hide shields with wooden frames were essential items of military equipment, a method to protect warriors against enemy arrows and spears.

a method to protect
as a method protecting
protecting
as a protection of
to protect
Choice C is best because the participle protecting begins a phrase that explains what the shields did. Choices A and B awkwardly use the singular word method to refer to items of military equipment rather than to the use of such items. Also, a method of protecting would be more idiomatic than a method to protect in A or a method protecting in B. In B and D, as is incorrect; also, a protection in D has no noun for which it can logically substitute. Choice E is incomplete; used to protect would have been acceptable.

As为什么不对呢?如果将d中的as换成like对么?

这个以前应该讨论过，mm参考一下总结帖吧。

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=27472&page=1