The young soldier really doesn't know ________ to stop the baby from crying hard.

1. A.
   what to do
2. B.
   how to do
3. C.
   when to do
4. D.
   where to do

The young soldier really doesn’t know       to stop the baby from crying hard.

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