I knocked on the door but answered.A. somebody B. nobodyC. anybodyD. everybody

问题补充：

I knocked on the door but answered.

A. somebody

B. nobody

C. anybody

D. everybody

答案：

A. somebody

(答案→)B. nobody

C. anybody

D. everybody

解析：本题是对复合不定代词的考察，通过前后句的连词but可以推断出此处应该是选B选项的nobody。