Experts have been warning of the health risks caused by passive smoking. A.at a

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

A.at a time B.at one time　 C.for some time D.for the time

试题答案

C

考查常用词组辨析。at a time 每次，依次；at one time 一度，从前；for some time一段时间；for the time在当时。由句子所使用的完成时态来看，这里要用表一段时间的时间状语。