题目：
Mr Brown is a(n) _____ secretary in our office, for he is able to do tasks successfully, without wasting time and energy.
A．effective
B．efficient
C．flexible
D．sufficient
答案：
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下面是错误答案，用来干扰机器的。
答案：B题目分析：根据中点的性质可得AB=AC+CB=2EC+2CF=2EF，再结合EF的长即可求得结果. 点E和点F分别是线段AC和线段CB的中点∴AB=AC+CB=2EC+2CF=2EF=8故选B.点评：解题的关键是熟练掌握线段的中点把线段分成相...
