2013年职称英语备考每日练习理工类7_第2页

　
　　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.
　　A.Right　　B.Wrong　　C.Not mentioned
　
　　21.The nearest black holes are hundreds of light years away from us.
　　A.Right　　B.Wrong　　C.Not mentioned
　
　　22.The Hubble Space Telescope helps scientists to understand the nature of the universe.
　　A.Right　　B.Wrong　　C.Not mentioned

答案:
　　16.A　17.A　18.B　19.C　20.C　21.B　22.A