问题补充：
how to work out the difficult physics problem ,he asked the teacher for help .
A. Not to know
B. Not knowing
C. Knowing not
D. Not known
答案：
A. Not to know
(答案→)B. Not knowing
C. Knowing not
D. Not known
解析：句意是：不知道怎么解决这个难的物理问题，他向老师求助。因为he和know是主动关系，用现在分词做状语，否定式是在现在分词前面加not，选B。
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