Now that there are still some problems remaining we have to work extra hours tonightA. s

问题补充：

Now that there are still some problems remaining , we have to work extra hours tonight

A. solved

B. solving

C. to be solved

D. to solve

答案：

A. solved

B. solving

(答案→)C. to be solved

D. to solve

解析：表示将来，故用不定式，且solve与其逻辑主语some problems之间是被动关系，故用不定式的被动式。