I really don’t know why his later discovery, ______ greater importance to science, is not so well known to the world.

　 A．which I think is of 　　　　 B．which I think it is of

　 C．of which I think it is 　　 D．I think which is of

I really don’t know why his later discovery, _________ greater importance to science, is not so well known to the world．

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