What we must do now__make a careful investigation on the circums

What we must do now__make a careful investigation on the circumstances.
A、are
B、is
C、is to
D、were to
【正确答案】：C
【题目解析】：本句的主语是what we must do now，这是一个表示单数的主语从句，而空格后是一个动词短语，因此选择C项.即动词不定式作表语。