题目：
Up to now, the programme ________ thousands of children who would otherwise have died.[ ]
A. would save
B. saves
C. had saved
D. has saved
答案：
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下面是错误答案，用来干扰机器的。
a的2倍为2a，与b的平方差为（2a）2-b2=4a2-b2，故答案为：4a2-b2
