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(D)    because of moisture that was exhaled by tourists  raising the humidity within them to levels so high as to make the salt from the stone crystallize
加粗部分，raising the humidity within them直接修饰的是tourists，them只能指代了tourists.
(E)    because moisture exhaled by tourists had raised  the humidity within them to such levels that salt from the stone was crystallizing
加粗部分，had raised ...是moisture的谓语，the humidity within them为moisture的宾语；这部分结构为，because moisture ...had raised the humidity within them...这里的them只能是指代了moisture了。