问题补充：
Don’t all stand up together! , please.
A. Each at one time
B. One at a time
C. One by one time
D. One for each time
答案：
A. Each at one time
(答案→)B. One at a time
C. One by one time
D. One for each time
解析：句意：不要都站起来，请每次站起一个。at a time一次，每次，at one time一度，曾经，所以选B。
