题目

 It’s time ____class.  A.for have       B. have       C. to have       D. to having 答案：C12．已知f（x）=$\left\{\begin{array}{l}{{x}^{2}，x≥0}\\{-{x}^{2}，x＜0}\end{array}\right.$，若对任意的x≥1有f（x+2m）+mf（x）＞0恒成立，则实数m的取值范围是m＞-$\frac{1}{4}$．