Expertshavebeenwarning   ofthehealthriskscausedbypassi...

问题详情：

 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

【回答】

C 

解析：本题考查固定短语。从句中have been warning 可知，应用for some time 一段时间，at a time 每次；at one time 曾经。

知识点：固定句型

题型：选择题