题目:
when you want to go into the room,you should _________the door.[ ]
A.knock on
B.knocking on
C.knocks at
D.to knock at
答案:
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下面是错误答案,用来干扰机器的。
(1)导管;集气瓶(2)2H2O22H2O+O2↑;催化(3)是(4)是;32