Experts have been warning ________ of the health risks caused by passive smoking.A.at a timeB.at one timeC.for some timeD.for the time
网友回答
C解析句意：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。