OG10-39 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
答案A
这里独立主格如果不按伴随状语理解的话，那就只能作原因状语了，表因为是个保护士兵的method才能是essential items of military equipment（但往往原因状语在主句前）