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21、

I was out of town at the time, so I don’t know exactly how it _______.    

A. was happening        B. happened       C. happens         D. has happened    

21、C

解析：考查代詞及其含義，由語意：他們必須面臨的其中一個問題就是公共衛(wèi)生問題，只有that,可以代替the problem.

請在這里輸入關鍵詞：
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