Question

We don’t know ______ it next. Let’s go and ask Mr Li

1. A.
   what to do
2. B.
   to do what
3. C.
   whether to do
4. D.
   to do whether

Answer 1

C
动词不定式作定语时要放在所修饰词的后面，whether to do是如何去做的意思，what to do是做什么的意思。It是我们做的对象，我们应是不知道是否去做，所以选C