Expertshavebeenwarning ofthehealthriskscausedbypassi...

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

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。