[讨论]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

我的问题是：1、ETS为什么说“a method of protecting would be more idiomatic than a method to protect in A”？to protect 放在 a method之后做定语表示目的不行吗？

2、B中的as为什么不对呢？看了以前的讨论还是不太明白。

谢谢！