导致X线衰减的原因是（） A.物质和距离 B.X线不可见 C.X线波长短 D.X线能
题型：单项选择题
问题：
导致X线衰减的原因是（）
A.物质和距离
B.X线不可见
C.X线波长短
D.X线能量大
E.X线是电磁波
题型：单项选择题
| 完形填空。 | |||
| We have known for a long time that flowers of different plants open and close at different times of day.This is so __1__ that there seems to be no need to ask the reason for it. Yet no one __2__ understands why flowers open and close like this at particular times. The process is not as __3__ as we might think,as recent experiments have shown. In one study, flowers were kept in constant __4__. We might expect that the flowers, without any information about the time of day, did not open as they __5_ do. As a matter of fact, they __6__ to open at their usual time. This suggests that they have some mysterious(神秘的) way of knowing the __7__. Their sense of time does not __8__information from the outside world; it is, so to speak, inside them, a kind of "inner clock". This __9__ may not seem to be very important. However, it was later found that not just plants but also __10__ , including man, have this "inner clock" which __11__ the working of their bodies and influences their activities. Men, then, are also influenced by this mysterious __12__. Whether we wish it or not, it affects such things in our life as our need for sleep, our need for food. In the past, this did not really __13__ because people lived in natural condition. In  the __14__ world, things are different; now there are spacemen, airplane pilots and, in ordinary life, a lot of people who have to work at night. It would be very __15_ , then, to know more about the "inner clock". Such things as flowers might help us understand more about ourselves. | |||
| ( )1. A. familiar ( )2. A. partly ( )3. A. complex ( )4. A. quietness ( )5. A. finally ( )6. A. refused ( )7. A. time ( )8. A. deal with ( )9. A. discovery ( )10.A. beasts ( )11.A. controls ( )12.A. world ( )13.A. happen ( )14.A. ancient ( )15.A. hard | B. strange B. personally B. simple B. darkness B. completely B. decided B. secret B. add to B. activity B. animals B. studies B. flower B. exist B. modern B. expensive | C. similar C. really C. mature C. loneliness C. physically C. continued C. process C. give away C. invention C. strangers C. measures C. power C. matter C. wonderful C. convenient | D. special D. willingly D. meaningful D. sadness D. normally D. failed D. study D. depend on D. method D. humans D. destroys D. experiment D. work D. peaceful D. important |