When the boy couldn't find his mother,he didn't know

问题描述:

When the boy couldn't find his mother,he didn't know
A.what to do B.how to do C.what to do it D.how to do it 要有解析

正选A
b中how to do ,do没有宾语 how 不能做宾语
c中 what to do it ,do的宾语重复 it 和what
d中 how to do it ,怎样做这件事 语义不通
重点 do 是及物动词