Experts have been warning of the health risks caused by pass

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Experts have been warning of the health risks caused by passive smoking
33.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
为什么选择C?

at a time 每次,一次（像吃药每次几片就可以用）
at one time 同时,曾经
for some time 已有一段时间
for the time 在当时
选C的理由很简单：句子用的是完成时态,所以得找一个能持续的时间表达方式, A,B,D都是点时间,不能用在完成时里.

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