Being alone in outer space can be frightening. That is one reason why astronauts on solo (单独的) space flights were given plenty of work to keep them 21 . They were also constant communication with people on the earth. 22 , being with people from whom you cannot get away might be even harder than being alone. This is what happens on long submarine (潜水艇) voyages. It will also happen on 23 space flights in the future. Will there be special problem of adjustment under such conditions? Scientists have studied the reactions of men to one another during long submarine voyages. They have found that the longer the voyage lasts, the more serious the problem of 24 is. When men are 25 together for a long period, they begin to feel uneasy. Everyone has little habits of speaking and behaving that are ordinarily acceptable. In the limited space over a long period of time, however, these little habits may become very 26 . Apparently, although no one wants to be 27 all the time, everyone needs some degree of privacy. When people are enclosed together, they are in what is called a stress situation. That means that they are under an unusual amount of 28 or stress. People who are well-adjusted are able to 29 stress situations better than others. That is one reason why so much care is taken in 30 our astronauts. These men undergo a long period of testing and training. One of the things tested is their behavior under stress. 21. A. tired                    B. asleep                 C. conscious            D. busy 22. A. So far                  B. After all              C. However             D. Therefore 23. A. long                    B. fast                     C. dangerous            D. direct 24. A. fuel                     B. entertainment       C. adjustment           D. health 25. A. shut up                B. held up               C. brought up          D. picked up 26. A. pleasing               B. annoying             C. common              D. valuable 27. A. noisy                   B. alone                  C. personal              D. sociable 28. A. emphasis              B. conflict               C. power                 D. pressure 29. A. handle                 B. create                  C. affect                  D. investigate 30. A. becoming             B. choosing             C. ordering              D. promoting