Expertshavebeenwarning   ofthehealthriskscausedbypassi...

問題詳情：

 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：本題考查固定短語。從句中have been warning 可知，應用for some time 一段時間，at a time 每次；at one time 曾經。

知識點：固定句型

題型：選擇題