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
网友回答
A. at a time
B. at one time
(答案→)C. for some time
D. for the time
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”。