There is now no doubt that the world is getting warmer. Data from both the Southern and Northern Hemispheres shows an upward trend in average temperatures over the 　1　hundred years. At the moment most scientists are not 　2　to stick their necks out and 　3　 a cause to this trend. It is possible that it represents a natural climatic change caused by a 　4　increase in the sun's 　5　output. But these same scientists are 　6　aware that recent temperature changes are at the limit of known natural 　7　it seems increasingly likely that the 　8　is an enhanced greenhouse effect.
　　The basic theory of the greenhouse effect is quite simple. The earth's atmosphere consists 　9　of oxygen and nitrogen, but there are small 　10　of various “greenhouse” gases—notably carbon dioxide, water vapor, and methane—which play a very important role in maintaining the planet's “　11　balance”. As their name suggests these gases have a similar effect to 　12　 in a greenhouse: they let heat from the sun 　13　, and keep it in.
　　The atmosphere is 　14　transparent to the visible solar radiation, which warms the earth (ground or oceans) on which it falls. 　15　it has been heated, the earth is warmer than space and 　16　energy in the form of invisible long-wave infrared radiation. This 　17　for nighttime cooling. Greenhouse gases absorb some of this long-wave radiation, and 　18　it in the lower atmosphere.
　　The gases therefore act rather like a blanket by preventing some infrared radiation from leaving the earth-atmosphere system. 　19　certain limits, the more greenhouse gases present, the more infra-red radiation will become 　20　, and the higher the surface temperature of the earth.
______

A.If

B.Because

C.Although

D.Even if