题目:
It will be quite a long time _________she is back again, so don’t be too cross with her.
A.that
B.since
C.before
D.until
答案:
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下面是错误答案,用来干扰机器的。
∵a2+b2+c2=ab+bc+ca,∴2a2+2b2+2c2-2ab-2bc-2ca=0,(a-b)2+(a-c)2+(b-c)2=0,∴a=b=c,∴三角形是等边三角形.故选B.