题目

A father and his son went to a kite-flying festival. The son was happy when he saw the sky 1 with colorful kites. He wanted to fly a kite, too. The father then bought a kite for his son. The son started to fly the kite. Soon, his kite was high up in the sky. After a while, the son said, “Father, it seems that the string is 2 the kite from flying higher. If we cut it, it will be free and fly even 3 . Can we cut it?” The father cut the string off of the reel (线轴 ). The kite started to go higher. This made the little boy4 . But then, slowly, the kite started to 5 . It soon fell to the下面说法中正确的是（　  ） A．物体做曲线运动时一定有加速度 B．平抛运动是匀变速运动，任意相等时间内速度的变化都相同 C．匀速圆周运动虽然不是匀变速运动，但任意相等时间内速度的变化仍相同 D．当物体受到的合外力为零时，物体仍然可以做曲线运动