问题补充：
how to do the homework, I went to ask my 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：考察非谓语动词.句意:不知道如何做作业,我去问我的老师寻求帮助.现在分词表示主语正在进行的伴随性动作,和句子主语是主谓关系,结合语境可知选B.
