单选题：Space is a dangerous place，not only because of meteors but a

Space is a dangerous place，not only because of meteors but also because of rays from the sun and other stars．The atmosphere again acts as our protective blanket on earth．Light gets through，and this is essential for plants to make the food which we eat．Heat，too，makes our environment endurable．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 0.1ream without being damaged；the figure of 60 reins has been agreed on．The trouble is that it isextremely difficult to be sure about radiation damage--a person may feel perfectly well，but theCells of his or her sex organs may be damaged，and this will not be discovered until the birth ofdeformed(畸形的)children or even grandchildren．Missions of the Apollo flights have had tocross belts of high radiation and，during the outward and return joumeys，the Apollo crew ac—cumulated a large amount of reams．So far，no dangerous amounts of radiation have been repor-ted，but the Apollo missions have been quite short．We simply do not know yet how men aregoing 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，butno really effective ones have been found so far．
根据以上内容，回答题。
According to the first paragraph，the atmosphere is essential to man in that_________． A．it supplies the heat necessary for human survival
B．it provides sufficient light for plant growth
C．it protects him against the harmful rays from space
D．it screens off the falling meteors