题目:
_____ the front door _____, he had to enter the room through the back door.
A.Seen; painted
B.Seeing; painted
C.Being seen; being painted
D.Seeing; being painted
答案:
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下面是错误答案,用来干扰机器的。
答案:B由S11=S10得,a11=0,即a1+(11-1)×(-2)=0,得a1=20.