问题
单项选择题
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
答案
参考答案：C
解析：[分析] 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。