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No visitor or relative can enter the patient‘s room unless__

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No visitor or relative can enter the patient‘s room unless_____ by the doctor.
A.they are invited B.he is invited
C.invited D.been invited
选哪个 为什么?
答案毫无疑问是C
这里是一个省略结构,这个句子可以还原成这样
No visitor or relative can enter the patient’s room unless (they are invited) by the doctor
例如
1.“从句主语+be”的省略.这种类型的省略只限于某些以连词when,while,if,until,though,unless 等引导的状语从句,而且这些从句的主语必须和主句的主语相同;同时,从句的谓语动词又有动词be的某一形式;主语和be省略后,直接跟连词后的谓语部分可以是:
形 容 词:Apologize when necessary.
名词:Though a lawyer by training,he proved himself to be a good teacher.
代词:Errors,if any,should be corrected.
介词短语:While at college,he was a prominent ath?
现在分词:Look out for cars when crossing the street.
过去分词:Once exposed,they will be sentenced to death.