I think is necessary us to do something useful to protect the birds.A. this; forB. it; o

问题补充：

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

A. this; for

B. it; of

C. this; of

D. it;for

答案：

A. this; for

B. it; of

C. this; of

(答案→)D. it;for

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