—I don’t know _______to do next?

  --It’s easy .Press the red button in 2 minutes.

A.what          B.how               C.where         D.when

I really don’t know_________.

A. when to do                    B. how to do               C. what to do            D. where to do

She looks very sad. I want to help her , but I just don’t know_________ to do.

 A. what     B. when   C. how      D. where