___ it carefully can we hope to find a way to solve the challenging problem.

A．When we observe                                       B．When to observe                         

C．That we should only observe                   D．Only by observing

________ it carefully can we hope to find a way to solve the challenging problem．

1. A.
   When we observe
2. B.
   When to observe
3. C.
   That we should only observe
4. D.
   Only by observing