The temperature of the sun is over 5000 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 the 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 10000 miles above the visible(可见的) surface and goes outward for millions of miles．This is the only part of the sun that can be seen during an eclipse (食) 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 (一束束穗状) 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．

1．Matter on the sun can exist only in the form of gas because of the sun’s ______．

A．size B．age C．location D．temperature

2．With what topic is the second paragraph mainly concerned?

A．How the Sun evolved．

B．The structure of the Sun．

C．Why scientists study the Sun．

D．The distance of the Sun from the planets

3．All of the following are parts of the sun’s atmosphere EXCEPT the ______．

A．corona B．chromosphere C．photosphere D．core

4．According to the passage, as the corona rays reach the planets, they become ______．

A．hotter B．clearer C．thinner D．stronger

5．The paragraphs following the passage most likely discuss which of the following?

A．The remaining layers of the sun．

B．The evolution of the sun to its present form．

C．The eclipse of February 1979．

Keys: 1-5 DBDCA