【题目】People often have difficulty facing reality when further scientific discovery challenges _______ they have previously thought to be settled facts.
A.whatB.thatC.whoD.which
在线课程【答案】A
【解析】
考查名词性从句。句意：当进一步的科学发现对他们之前认为是既定事实的东西发起挑战时，人们通常很难面对现实。分析句子可知，空白处做challenges（挑战）的宾语，且引导宾语从句，在从句中做thought（认为）的宾语，称之为“双重成分”，用what。故选A项。