The overseas Chinese scientist made another wonderful discovery, ______ of great value to study AIDS.

1. A.
   which I think it is
2. B.
   which I think is
3. C.
   I think it is
4. D.
   I think is

The overseas Chinese scientist made another wonderful discovery, ___________ of great value to study AIDS.

 A．which I think it is        B．which I think is    C．I think it is          D．I think is