to be falling. Scientists conclude, therefore, that up to now natural influences on the weather have exceeded those caused by man. The question is: Which natural cause has the most effect on the weather?

　　One possibility is the behavior of the sun. Astronomers at one research station have studied the hot spots and "cold" spots, that is, the relatively less hot spots, on the sun. This seems to have a greater effect on the distribution of the earth's atmospheric pressure, and consequently on wind circulation (循环). The sun is also variable (可变的) over a long term. Its heat output (输出) goes up and down in cycles. The latest trend being downward is on the way. But that is only an imagination.

　　9．The carbon dioxide in the atmosphere would ________.

　　A．prevent the sun's rays from reaching the earth's surface

　　B．mean a warming up in the Arctic

　　C．account for great changes in the climate

　　D．raise the temperature of the earth's surface

　　解析：选D　细节理解题。文章第一段讲到二氧化碳在大气中吸收太阳的光线并阻挡光热的流失，文章第二段最后一句话又再次提到二氧化碳能使地球变热的理论。因此，答案为D项。

　　10．The article was written to explain ________.

　　A．the greenhouse effects

　　B．the solar effects on the earth

　　C．the solar weather

　　D．the causes affecting weather

　　解析：选D　主旨大意题。全文讲述的是影响天气的诸多原因。选项A、B、C在文中均被提及但不足以概括全文的主旨，只有D项是比较全面的。

　　11．Although the fuel is greater in the northern part, temperatures there seem to be falling. This is ________.

　　A．mainly because the carbon dioxide is rising

　　B．possibly because the ice caps in the poles are melting

　　C．due to the effect of the earth's climate

　　D．possibly due to the activity of the sun

解析：选D　细节理解题。文章第三段提及这一现象时讲到，科学家们认为：这一现