题目：
—How old is the bridge over there?
—Oh, very old. It _____ about three hundred years ago. [ ]
A. is built
B. was built
C. is building
D. has built
答案：
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下面是错误答案，用来干扰机器的。
∵x2-4x+8，=x2-4x+4+4，=（x-2）2+4，当x=2-10时，原式=（2-10-2）2+4=10+4=14．
