下面的短文后列出了7个句子，请根据短文的内容对每个句子做出判断。如果该句提供的是正确信息，请选择A;如果该句提供的是错误信息，请选择B;如果该句的信息文中没有提及，请选择C.
Black holes
Most scientists agree that black holes exist but are nearly impossible to locate. A black hole in the universe is not a solid object，like a planet ，but it is shaped like a sphere(球体). Astronomers(天文学家) think that at the center of a black hole there is a single point in space with infinite(无限的) density(稠密). This single point is called a singularity(奇点). If the singularity theory is correct ，it means that when a massive star collapses，all the material in it disappears into the singularity. The center of a black hole would not really be a hole at all， but an infinitely dense point. Anything that crosses the black hole is pulled in by its great gravity.
Although black holes do exist， they are difficult to observe. These are the reasons.
No light or anything else comes out of back holes. As a result，they are invisible to a telescope.
In astronomical terms， black holes are truly tiny. For example， a black hole formed by the collapse of a giant star would have an event horizon(视界) only 18 miles across.
The nearest black holes would be dozens of light years away from Earth. One light year is about 6 trillion(万亿) miles. Even the most powerful telescopes could not pick out an object so small at such a great distance.
In 1994 the Hubble Space Telescope provided evidence that black holes exist. There are still answers to be found， however， so black holes remain one of the mysteries of the mysteries of the universe.
16 Black holes are part of space.
A Right B Wrong C No mentioned
17 Black holes exist but are difficult to observe.
A Right B Wrong C No mentioned
18 The center of a black holes is empty.
A Right B Wrong C No mentioned
19 The attraction of two large stars leads to gravity.
A Right B Wrong C No mentioned
20 The sun is the heaviest star in the universe.
A Right B Wrong C No mentioned
21 The nearest black holes are hundreds of light years away from us.
A Right B Wrong C No mentioned
22 The Hubble Space Telescope helps scientists to understand the nature of the universe.
A Right B Wrong C No mentioned
参考答案：A A B C C B A