State media said the power failure knocked out an electronic safety system that________the second train to the problem.

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问题 State media said the power failure knocked out an electronic safety system that________the second train to the problem.

选项 A、should have alerted
B、should be alerted
C、had alerted
D、had been alerted

答案A

解析 本题考查虚拟语气。由国家媒体发布的内容可知,停电导致电子安全系统瘫痪,that引导的定语从句修饰“an electron safety system(电子安全系统)”。根据选项中的alert判断,此系统本该向第二辆火车发出警报,但因为停电实际上并没有发出警报,因此应该采用should have done的形式,故A项为答案。should have done表示“本应该做而实际上没有做”。
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