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

题型：不详难度：来源：

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

答案

C

解析

for some time 意为“一段时间”，因为谓语用了现在完成进行时，所以要用for some time，表示从过去的某个时候开始一直延续到现在的一段时间内，专家一直警告说被动吸烟引起健康危急。at a time意为“每次,在某时”；at one time意为“同时,曾经”；for the time意为“暂时”。