Nowadays, there is a _____ increase in children's creativity, for they are greatly encouraged to develop their

talents. [ ]

A. sharp

B. slight

C. natural

D. modest

质量是m的物体在粗糙的水平面上受水平恒定拉力F的作用，加速度大小为a，为了使加速度变为2a，下列那些方法可行的是（　　）

A．力F增加为原来的二倍

B．质量减小为原来的二分之一倍

C．力F和质量都增加为原来的二倍

D．力F和动摩擦因数都增加为原来的二倍