专四
A．must be
B．had been
C．could be
D．must have been
情态动词must的用法。must be表示对现在的猜测，must have been表示对过去的猜测，故选D。选项C容易造成干扰，从现在猜测过去，指“可能”时可用“could+ have done”，而选项C用的是could be，故不对。
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