单选题 21.I advised her that in the presence of the princess she should not say anything until______.
【正确答案】 A
【答案解析】until可以引导省略句，此处省略了she was。until从句的主语必须与主句的主语相同，谓语必须是be动词才可以用省略句，只保留表语。until后面不跟动名词和不定式。