单选题 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 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 corona is a brilliant, pearly white, firmly 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 weak 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 speed 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 There may be a solid core at the center of the Sun ______.

单选题 With what topic is the passage mainly concerned?

单选题 All of the following are parts of the Sun's atmosphere EXCEPT the ______.

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

单选题 The paragraphs following the passage most likely discuss which of the following ______.