问题
选择题
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
答案：B
题目分析：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。