问题补充：
For some time the expert people of the health risks caused by passive smoking. He says he won’t stop until all the smokers around him decide to quit smoking.
A. is warned
B. is warning
C. has been warning
D. has been warned
答案：
A. is warned
B. is warning
(答案→)C. has been warning
D. has been warned
解析：考查时态：句意是：有一段时间这个专家一直在警告被动吸烟引起的健康危害，他说知道身边所有的抽烟者都戒烟了他才停止。由此可知警告这个动作是从过去到现在一直在做，而且还要继续，符合现在完成进行时的用法。选C。
