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.
30. 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
选择C没有问题,但是OG中队E的解释有些不明白“Although the infinitive to protect would work if it were not preceded by a comma, it cannot act as a nonrestrictive adjectival phrase modifying items.”,意思是说就算前面没有逗号, to protect也不能做items的非限制性形容短语?