题目

They ______ busy doing their homework.A. is B. am C. are D. / 答案：【答案】C【解析】句意：他们正忙于写作业。主语they是复数形式，系动词应用are。故选C项。（2013•镇江二模）已知a为正的常数，若不等式1+x≥1+x2-x2a对一切非负实数x恒成立，则a的最大值为88．