Expertshavebeenwarning ofthehealthriskscausedbypassi...

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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。