c．are being tested d．have been tested
解析： 考查动词的时态。根据问句中的what’s that terrible noise 可知，这里应该用进行时态，噪音是机器正在被检测时发出的，所以用现在进行时的被动语态。
答案： c
2．(2011·北京西城模拟)the attackers were arrested and didn’t know where they________.
a．would take b．are taken
c．were being taken d．will be taken
解析： 句意为：袭击者被捕了，他们不知道正被带往何处。根据didn’t know 可知，空格处也应该用过去的某种时态，所以b、d不正确；根据句意可知此处用被动语态，故选c。
答案： c
3．(2011·南京模拟)people begin to know that most environmental problems exist because necessary measures for preventing them__资源难易程度:★★★★★★★★★★★★★★★
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