Radiation is the greatest known danger to explorers in space. The unit of radiation is called “rem”. Scientists have reason to think that a man can put up with far more radiation than 0.1 rem without being damaged; the figure of 60 rems has been agreed on. The trouble is that it is extremely difficult to be sure about radiation damage-a person may feel perfectly well, but the cells of his or her sex organs may be damaged, and this will not be discovered until the birth of deformed（畸形的）children or even grandchildren. Missions of the Apollo flights have had to cross belts of high radiation and, during the outward and return journeys, the Apollo crew accumulated a large amount of rems. So far, no dangerous amounts of radiation have been reported, but the Apollo missions have been quite short. We simply do not know yet how men are going to get on when they spend weeks and months outside the protection of the atmosphere, working in a space laboratory. Drugs might help to decrease the damage done by radiation, but no really effective ones have been found so far.
51. According to the first paragraph, the atmosphere is essential to man in that ______.
A) it protects him against the harmful rays from space
B) it provides sufficient light for plant growth
C) it supplies the heat necessary for human survival
D) it screens off the falling meteors
52. We know from the passage that ________.
A) exposure to even tiny amounts of radiation is fatal
B) the effect of exposure to radiation is slow in coming
C) radiation is avoidable in space exploration
D) astronauts in spacesuits needn’t worry about radiation damage
53. The harm radiation has done to the Apollo crew members _______.
A) is insignificant
B)seems overestimated
C) is enormous
D) remains unknown
54. It can be inferred from the passage that ________.
