[单项选择]Space is a dangerous place, not only because of meteors(流星) but also (1)_____ rays from the sun and other stars. The atmosphere again (2)_____ as our protective blanket on earth. Lightgets through, and this is essential for plants to (3)_____ the food which we eat. Heat, too, makes our environment endurable. Various kinds of rays come through the air (4)_____ outer space, but enormous quantities of radiation from the sun are (5)_____ off. As soon as men leave the atmosphere they are (6)_____ to this radiation, but their space suits or the walls of their spacecraft, if they are inside, (7)_____ prevent a lot of radiation damage. (8)_____ is the greatest known danger to explorers in space. The unit of radiation is called "rem". Scientists have (9)_____ to think that a man can (10)_____ far more radiation than 0.@1 ram without being damaged: the figure of 60 rems has been agreed on. The trouble is that it is extremely difficult to make (11)_____ about radiation damage—a person may feel perfectly well, (12)_____ the cells of his or her sex organs may be damaged, and this will (13)_____ be discovered until the birth of deformed 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 (14)_____ a large amount of rems. So far; no (15)_____ 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 (16)_____ when they spend weeks and months outside the (17)_____ of the atmosphere, (18)_____ in a space laboratory. Drugs might help to (19)_____ the damage done by radiation, but no really (20)_____ ones have been found so far.
A. yet
B. still
C. nor
D. continuously