[单项选择] 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
A. The remaining layers of the Sun.
B. The evolution of the Sun to its present form.
C. The eclipse of February 1979.
D. The scientists who study astronomy.

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