问题补充：
—I have been waiting for you so long.
—Sorry, I have been in the library, my friend look for some books.
A. helped
B. helping
C. to help
D. help
答案：
A. helped
(答案→)B. helping
C. to help
D. help
解析：考察非谓语动词，根据句子结构可知，此处为状语，help的逻辑主语是I，help与逻辑主语I之间是主动关系，即选择B。
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