阅读下列短文，从每题所给的（A、B、C和D）四个选项中，选出最佳选项。

　　To control weather over large areas of the world wonldseem,at this time,to be impossible．However,man has beenhighly 　　1　　 in his attempt to change the weather on a very 　　2　　 scale（规模）．He has 　　3　　 microclimate inside andoutside homes and offices．

　　Micro means small,and microclimate 　　4　　 the climateconditions over a small area．There are many things that might 　　5　　 the climate in a small area to be different from the 　　6　　 climate of the region where it is located．

　　　　7　　,within cities there might be smaller areas wherethe climate is different．The microclimate in the yard of one home may be 　　8　　 different from the one in the yard acrossthe street．It is the 　　9　　 of trees and their position in a yardthat 　　10　　 the microclimate．One could probably name many 　　11　　 things that would change the microclimate of a yard．High-altitude（高空的）flying and 　　12　　 development in spaceflight present situations which 　　13　　 complete control of microclimates．Before 　　14　　 planes can fly in the air of the stratosphere（同温层）,the cabins（机舱）have to be pressurized．　　15 　　passengers can obtain the necessary oxygen．

　　In space 　　16　　,there is no air．Astronauts have to take their air with them．　　17　　 have been developed in which this air can be 　　18　　 used over and over．In space,astronauts also have to be 　　19　　 from harmful radiation．The chief problem to be solved 　　20　　 man can exist in space is that of providing a microclimate in which he can survive．

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