单选题 Good pencil erasers are soft enough not ______ paper but hard enough so that they crumble gradually when used.
A. by damaging B. so that they damage
C. to damage D. damaging

【正确答案】 C
【答案解析】[解析] 语法应用。
“enough+不定式”(足够……能)结构常置于形容词后作状语，表示结果，本句是这一结构的否定式：所以C是答案。