阅读理解
The temperature of the Sun is over 5, 000 degrees Fahrenheit at the surface, but it rises to perhaps more than 16 million degrees at the center. The Sun is so much hotter than the Earth that matter can exist only as a gas, except at the core. In the core of the sun, the pressures are so great against the gases that, despite high temperature, there may be a small solid core. However, no one really knows, since the center of the Sun can never be directly observed.
Solar astronomers do know that the Sun is divided into five layers or zones. Starting at the outside and going down into the Sun, the zones are the corona, chromosphere, photosphere, convection zone, and finally the core. The first three zones are regarded as the Sun''s atmosphere, But since the Sun has no solid surface, it is hard to tell where the atmosphere ends and the main body of the Sun begins.
The Sun''s outermost layer begins about 10,000 miles above the visible surface and goes outward for millions of miles. This is the only part of the Sun that can be seen during an eclipse such as the one in February 1979. At any other time, the corona can be seen only when special instruments are used on cameras and telescopes to shut out the glare of the Sun''s rays.
The corona is a brilliant, pearly white, filmy light, about as bright as the full Moon. Its beautiful rays are a sensational sight during an eclipse. The corona’s rays flash out in a brilliant fan that has wispy spikelike rays near the Sun''s north and south poles. The corona is thickest at the sun''s equator.
The corona rays are made up of gases streaming outward at tremendous speeds and reaching a temperature of more than 2 million degrees Fahrenheit. The rays of gas thin out as they reach the space around the planets. By the time the Sun''s corona rays reach the Earth, they are weak and invisible.
单选题
Matter on the Sun can exist only in the form of gas because of the Sun''s ________.
【正确答案】
D
【答案解析】细节题。根据文章第2句：The Sun is so much hotter than the Earth that matter can exist only as a gas，except at the core．(太阳要比地球热得多，以致除了核心以外，物质只能以气体状态存在。)所以D是正确答案。
单选题
With what topic is the second paragraph mainly concerned?
【正确答案】
B
【答案解析】主旨大意题。第二段的第一句就是主题句：Solar astronomers do know that the Sun is divided into five layers or zones．(太阳天文学家知道，太阳分为五层或五个地区。)接着，讲述从外向内这五个层的名称。所以这段是关于太阳的结构。故B是正确答案。
单选题
All of the following are parts of the Sun''s atmosphere EXCEPT the ________.
【正确答案】
D
【答案解析】细节题。根据第二段讲太阳从外向内分为日冕(corona)、火焰(chromosphere)、白热气体(photo- sphere)、对流层(convection zone，)和核心(core．)五层。前三层是太阳的大气层(The first three zones are regarded as the Sun''s atmosphere)。所以选项D是正确答案。
单选题
According to the passage, as the corona rays reach the planets, they become ________.
【正确答案】
C
【答案解析】细节题。根据文章最后一段：日冕的光线是由以巨大速度向外流淌的高温气体构成……，当这些气体光线到达行星周围的空间时就变得稀薄了。(The corona rays are made up of．．．The rays of gas thin out as they reach the space around the planets．)可知，C是正确答案。
单选题
The paragraphs following the passage most likely discuss which of the following?