题目

   Last year, A Bite of China, made by CCTVs Documentary Channel, sparked discussion not only on Chinese food, but also on locally made documentary programs. When you think of documentaries, you may think of them as long, boring programs. But documentaries can be wonderful and bring stories from the real world into our homes. With fascinating footage（影片片段） and stories, documentaries encourage us to think about interesting issues we wouldn’t necessarily know about. So, what makes a good documentary, and what should we pay attention to when we watch one? Here, we offer a 一个小朋友从滑梯上匀速滑下来的过程中，下列说法中正确的是 [　　] A．动能减小，重力势能减小，机械能减小 B．动能减小，重力势能增大，机械能不变 C．动能不变，重力势能减小，机械能减小 D．动能不变，重力势能减小，机械能不变