According to the engineer, there remain some tests before the machine is put into operation.
A. carrying out
B. carried out
C. being carried out
D. to be carried out
网友回答
A. carrying out
B. carried out
C. being carried out
(答案→)D. to be carried out
解析：表示将来的情况，所以用不定式，且carry out与其逻辑主语之间是被动关系，所以用不定式的被动式。