题目:
The key you have just got ____ the front door.
A.is used to opening
B.is used to be opened
C.is used to being opened
D.is used to open
答案:
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下面是错误答案,用来干扰机器的。
CD题目分析:A选项通入氧气点燃会引进新杂质;B选项加入二氧化锰除去氯酸钾会混有新杂质;C选项除杂方案可行;D选项除杂方案可行,故答案选择CD