It is not yet well understood _______ has made the rainforest disappear year by year.
     A. what         B. which         C. that            D. how

It is not yet well understood _______ has made the rainforest disappear year by year.
A. what         B. which         C. that            D. how

It is not yet well understood ________ has made the rainforest disappear year by year．

1. A.
   what
2. B.
   which
3. C.
   that
4. D.
   how

It is not yet well understood _______ has made the rainforest disappear year by year.

　　 A. what　　　　 B. which　　　　 C. that　　　　　 D. how