Question

I think ______ is necessary ______ us to do something useful to protect the birds.

1. A.
   this; for
2. B.
   it; of
3. C.
   this; of
4. D.
   it;for

Answer 1

D
考查固定句型。It is+形容词+for sb. to do sth.句中it作形式主语。