一个中子和一个质子相结合生成一个氘核，若它们的质量分别是m1、m2、m3，则（ ）
A．由于反应前后质量数不变，所以m1+m2=m3，反应前后没有能量变化
B．由于反应时释放出了能量，所以m1+m2>m3
C．由于反应在高温高压下进行，从而吸收能量，所以m1+m2<m3
D．反应时放出的能量为（m1+m2－m3）c2（其中c是真空中的光速）
BC
解答本题需要掌握：核反应方程要遵循质量数和电荷数守恒；聚变反应后质量减小，放出能量；正确利用质能方程求释放的能量；掌握光子能量、频率、波长、光速之间关系．
解：该核反应方程质量数不守恒；聚变反应中的质量亏损△m=（m1+m2）-m3；聚变反应中亏损的质量转化为能量以光子的形式放出，故光子能量为E=（m1+m2-m3）c2；
点评：爱因斯坦质能方程为人类利用核能打开了大门，要正确理解质能方程中各个物理量是含义．
| 阅读理解。 |
| It was reported that over 300 million people in China were suffering from short-sightedness (近视眼). As more children suffer from eye trouble, their parents hope to cure the disease with medicine equipment by correcting the way children eat and sit. But in fact the cure given to one's eyes should start with the exercise on one's feet. Three methods to protect eyes are as follows. Firstly, don't fasten your shoes too tightly. Try wearing loose socks or walking bare-footed (赤脚) at home. When traveling, try wearing cloth shoes. Secondly, walking on tiptoes (踮着脚) can help improve one's eyesight and prevent short-sightedness. It's said that few ballet performers are short-sighted. Thirdly, rope skipping is good for one's eyesight. When skipping, one has to react quickly, which excites the brain as well as the eyes. It may also help make one grow taller. Besides foot exercise, it is good to pull one's ears. Pulling the ear 20 times is a good way to reduce the pressure on one's eyes. |
| 1. What's the best way to protect one's eyes? |
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| A. Curing the disease with medicine. B. Using the equipment to correct the way children read and sit. C. Starting with the exercise on one's feet. D. Taking showers very often. |
| 2. The undeline word "tightly" means ______ in Chinese. |
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| A. 松地 B. 紧地 C. 长的 D. 短的 |
| 3. Fom the passage we know ______ aren't often suffeing from short-sightedness. |
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| A. people taking the medicine B. people using the equipment C. people wearing tight socks D. people performing ballet |
| 4. Rope skipping can help one ______. |
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| A. grow taller B. react slowly C. sleep well D. walk fast |
| 5. What's the best title for the passage? |
[ ] |
| A. How to cue shot-sightedness B. Ways to protect eyes C. Foot exercise D. Pulling one's ears |