一蹦极运动员身系弹性蹦极绳从水面上方的高台下落，到最低点时距水面还有数米距离.假定空气阻力可忽略，运动员可视为质点，下列说法正确的是( )
A．运动员到达最低点前重力势能始终减小
B．蹦极绳张紧后的下落过程中，弹性力做负功，弹性势能增加
C．蹦极过程中，运动员、地球和蹦极绳所组成的系统机械能守恒
D．蹦极过程中，重力势能的改变与重力势能零点的选取有关
ABC
题目分析：A、运动员到达最低点前，重力对运动员一直做正功，运动员的重力势能始终减小．故A正确．
B、蹦极绳张紧后的下落过程中，弹力方向向上，运动员的位移向下，弹性力对运动员做负功，弹性势能增加．故B正确．
C、以运动员、地球和蹦极绳所组成的系统，只有重力和弹力做功，系统的机械能守恒．故C正确．
D、重力势能的改变与重力做功有关，取决于初末位置的高度差，与重力势能零点的选取无关．故D错误．故选ABC．
My father died when I was a baby, and my mother had to go out to work. I was the only 36 so I had no brothers and sisters to play with at home. I used to play in the street with other children, or ride my bike down to the park, but I remember I 37 a lot of time just sitting at home reading. We had a television, I think, but don’t remember 38 it very much. My grandparents lived with us and we used to do a lot of things together.I remember we played cards a lot. At the weekends, my mother always 39 -- We often went down to the sea and swam, and I 40 to swim when I was quite young. We didn’t have a car, so we went everywhere by bus. Occasionally（偶尔），one of my uncles used to come and take us out in his car, which was a great 41 .For holidays we always went to the seaside, but never far away maybe a short train journey. My mother wasn’t very 42 , and I don’t remember her ever 43 me. In fact（事实上），I got my way too much. I was given small pocket money because we weren’t very 44 and I probably spent most of it on sweets, as far as I remember I don’t think anyone thought they were 45 for you then.
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