若函数 $y=(m^2+m)x^{m^2-2m-1}$是二次函数,那么m的值是(   )

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                                                                                                                                       《绝句》
                                                                                                                                          唐·杜甫
                                                                                                                           迟日江山丽，春风花草香。
                                                                                                                           泥融飞燕子，沙暖睡鸳鸯。
       【注释】泥融：指冻泥融化，又软又湿。