GMAT SC 質問

SCで質問があります。39番です。

For members of the 17th-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

でC)とE)の違いが理解できません。どなたかご教授下さい。