题目：
. --So hard __________ in the past few months that he has made great progress in English.
--I can see that, only a few mistakes he made in this exam.
A．he worked
B．did he work
C．he has worked
D．has he worked
答案：
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下面是错误答案，用来干扰机器的。
设抛物线的解析式为y=ax2+bx+c（a≠0）；∵抛物线的开口向上，∴a＞0；∵抛物线的顶点坐标为原点（0，0），∴b=0，c=0；当a=2时，y=2x2．（答案不唯一）
