Although black holes do exist,they are difficult to observe.These are the reasons.

　　● No light or anything else comes out of black holes.As a result,they are invisible to a telescope.

　　● In astronomical terms,black holes are truly.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 universe.

　　16.Black holes are part of space.

　　A.Right　　B.Wrong　　C.Not mentioned

　　17.Black holes exist but are difficult to observe.

　　A.Right　　B.Wrong　　C.Not mentioned

　　18.The center of a black hole is empty.

　　A.Right　　B.Wrong　　C.Not mentioned

　　19.The attraction of two large stars leads to gravity.

　　A.Right　　B.Wrong　　C.Not mentioned

　　20.The sun is the heaviest star in the universe.

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