问题
选择题
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
答案
答案：C
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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
答案：C