.We will learn how to operate the machine .A. at a time B. at one timeC. at the timeD. in

问题补充：

.

We will learn how to operate the machine .

A. at a time

B. at one time

C. at the time

D. in time

答案：

A. at a time

B. at one time

C. at the time

(答案→)D. in time

解析：考查短语辨析。at a time每次，at one time 曾经，in time 及时，准时，句意：我们将学会如何准时操作机器。所以选D。