Space is a dangerous place, not only because of meteors(流星), but also because of rays from the sun a

Space is a dangerous place, not only because of meteors(流星), but also because of rays from the sun and other stars. The atmosphere again acts as our protective blanket on the earth. Light gets through, and this is necessary for plants to make the food which we eat. Heat, too, makes our environment suitable to live. Various kinds of rays come through the air from outer space, but enormous quantities of radiation from the sun are screened off. As soon as men leave the atmosphere they are exposed to this radiation but their spacesuits or the walls of their spacecraft, if they are inside, do prevent a lot of radiation damage.
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.
49．According to the first paragraph, the atmosphere is necessary to man because of the following reason EXCEPT________
A．protecting him against the harmful rays from space
B．providing sufficient light for plant growth
C．suppling the heat necessary for human survival
D．screening off the falling meteors
50．We learn 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
51．The harm radiation that has been done to the Apollo crew members ________
A．is insignificant   B．is enormous   C．seems overestimated  D．remains unknown
52．It can be inferred from the passage that________
A．the Apollo mission was very successful