Question

Experts have been warning ________ of the health risks caused by passive smoking.

1. A.
   at a time
2. B.
   at one time
3. C.
   for some time
4. D.
   for the time

Answer 1

C
句意：专家们提醒注意被动吸烟给健康带来的危险已有一段时间了。for the time暂时，眼下，故排除D；at a time=on each occasion每次，逐一，依次；不合题意，排除A；at one time=at a period of time in the past一度，曾经；不合题意，排除B；for some time=for a fairly long period of time一段时间；所以只有选C。