在物理学研究中，有时可以把物体看成质点，以下说法正确的是（ ）
A．研究乒乓球的旋转，可以把乒乓球看成质点
B．研究奥运冠军郭晶晶的优美跳水动作时，可把郭晶晶视为质点
C．研究神舟七号飞船绕地球运动的周期时，飞船可视为质点
D．研究车轮的转动，可以把车轮看成质点
答案：C
当物体本身的大小对所研究问题的影响可以忽略不计时，物体可以看成质点。研究乒乓球的旋转，不可以把乒乓球看成质点，A错。研究跳水动作时，郭晶晶不可视为质点，B错。研究飞船绕地球运动的周期时，飞船可视为质点，C对。研究车轮的转动，不可以把车轮看成质点，D错。
Two brothers worked together on a farm. One was married and had a large 41 .The other had no wife yet. Although they lived in different houses, they shared everything they got from their farm. One day, the single brother said to himself, “It’s not 42 that we share all the rice. I’m 43 and I need less than my brother.” So, every night he took a bag of rice from his store-room and 44 across the field 45 their houses, putting it into his brother’s store-room. At the same time, the married brother thought, “It’s not fair to share all the rice. I’m married and I have my wife and children to 46 me when I am 47 . However, my brother has no one to take care of his 48 .” So, each night he also took a bag of rice and put it into his brother’s store-room. Several months passed by. They found 49 strange that their rice never became less. Then, one dark night the two brothers ran into each other. Slowly they began to understand what was happening. Their bags fell onto the ground and tears 50 their eyes at once.
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