题目

When I was in my fourth year of teaching, I was a high school track coach (田径教练). One year, I had a student, John, who changed to our school from Greece, and seemed to be interested in sports, so I encouraged him to join our track team. I explained to him that even though he had never taken part in it before, I did believe that he could do well in any event, and l would be willing to teach him at whichever ones interested him. He accepted the offer, and began to work hard at every practice.   About a month later, I had found out from other ways that John was a first-class tenni如图所示，杂技演员在表演“水流星”节目时，用细绳系着的盛水的杯子可以在竖直平面内做圆周运动，甚至当杯子运动到最高点时杯里的水也不流出来．下列说法中正确的是（　　）A．在最高点时，水对杯底一定有压力B．在最高点时，盛水杯子的速度一定不为零C．在最低点时，细绳对杯子的拉力充当向心力D．在最低点时，杯中的水不只受重力作用