A
do是及物动词,必须带宾语或逻辑宾语.四个选项中只有疑问代词what可以做宾语.其它三个都是疑问副词,只能做状语（why作原因状语,how作方式状语,when作时间状语）.
顺便说一下：
疑问代词有who,which,what,whose,whom五个,语法功能相当于名词.who,which,what,在句子中可以做主语、宾语,whom只能做宾语,whose,which可以做定语.
疑问副词有when,where,how,why这么几个,语法功能相当于副词.都只能作状语,修饰动词.
The firemen know ______ to do.( ) A.what B.why C.how D.when
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