问题
多选题
已知212Bi的半衰期为1个小时，关于衰变下列说法正确的是（ ）
A．半衰期是指放射性元素的原子核有半数发生衰变所需的时间
B．α粒子就是氦原子核，α粒子电离本领很大，不宜用来进行金属探伤
C．现在有800个212Bi原子，经过4个小时后，未衰变的212Bi原子数为50个
D．β衰变中放出的电子，是核外电子获得能量后，被电离出来的
答案
A、半衰期是原子核有半数发生衰变的时间，具有统计意义，对大量的原子核适用，由原子核内部因素决定，与原子所处的物理环境和化学状态无关．故A、E正确，C错误．
B、α粒子就是氦原子核，α粒子电离本领很大，穿透本领很弱，不宜用来进行金属探伤．故B正确．
D、β衰变中放出的电子是原子核中的一个中子转变为一个电子和一个质子，电子释放出来，不是来自核外电子．故D错误．
故选：ABE．
完形填空
| 完形填空。 | |||
| Spend time in a bookshop can be enjoyable. If you go to a good shop, no __1__ will come near to you and say, "Can I help you?" You __2__ buy anything you don't want. You may try to find out __3__ the book you want is. But if you fail, the assistant will lead (引导) you there and then he will go away. It seems that he is not __4__ in selling any books at all. There is a story which tells us about a good shop. A medical student __5__ a very useful book in the shop, __6__ it was too expensive for him to buy. He couldn't get it from the library, either. So every afternoon, he went there to read __7__ at a time. One day, however, he couldn't find the __8__ from its usual place and was leaving when he saw an assistant signing to him. To his surprise, the assistant __9__ to the book in a corner, "I put it there so as not to be __10__ out." said the assistant. Then he left the student go on with his reading. | |||
| ( ) 1. A. assistant ( ) 2. A. won't ( ) 3. A. what ( ) 4. A. surprised ( ) 5. A. bought ( ) 6. A. but ( ) 7. A. little ( ) 8. A. book ( ) 9. A. went ( ) 10. A. sold | B. waiter B. needn't B. which B. surprising B. found B. and B. a little B. shop B. used B. taken | C. police C. mustn't C. where C. interesting C. read C. so C. few C. assistant C. pointed C. found | D. teacher D. can't D. when D. interested D. took D. or D. a few D. shopkeeper D. left D. watched |