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. a method to protect
B. as a method protecting
C. protecting
D. as a protection of
E. to protect
 
 
1. 修饰 ：(一级排除A,B,D, E )
句末的分词逻辑主语考虑; 不定式不能单独在句末做修饰成分、
the participle protecting begins a phrase that explains what the shields did
句末分词短语的逻辑主语优先考虑句子的主语(最后一句显然是要修饰盾牌的，除了c之外，其他的都修饰不到这儿)。
 
2. 单复数
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. (单复数问题)

3. 词用法
a method of 正确，，， a method to / a method doing 不正确
a method of protecting would be more idiomatic than a method to protect in A or a method protecting in B

4. 同位语： 同位语要考虑是否逻辑意思内涵一致
a protection in D has no noun for which it can logically substitute.