如图所示,一根轻绳跨过固定斜面顶端的定滑轮后系在一质量较大的球上,球的大小不可忽略。在轻绳的另一端施加一个力 F ,使球沿斜面由图示位置缓慢上升至斜面顶端,各处的摩擦不计,在这个过程中拉力 F ( )
A.
逐渐增大
B.
保持不变
C.
先增大后减小
D.
先减小后增大
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