问题补充：
Has how many spies we caught during the war up to now?
A. it been declared; have been set free
B. it declared; have been set free
C. they declared; has been set free
D. they declared; have set free
答案：
(答案→)A. it been declared; have been set free
B. it declared; have been set free
C. they declared; has been set free
D. they declared; have set free
解析：本题在考查对句子结构的分析能力的同时考查主谓一致原则，首先排除C、D，然后从句子结构分析可知这是个主语从句，由it做形式主语，而且是it is declared…句型，只是以疑问句形式出现增加了难度，真正的主语是how many引导的从句，从句中的时间状语up to now“到目前为止”，应该用现在完成时。当主语是个较长的句子或短语时，常用it作形式主语，而把真正的主语放在后面，但依然是主语从句，如：Does it make any difference where we shall have the meeting?
