浪费钢材是拱桥最重要的缺点。()

A. employ
B. observe
C. remove
D. instruct

A. Is there proof that black holes really exist？
B. What are different types of black holes？
C. How are black holes formed？
D. How were black holes named？
E. What happens to the objects around a black hole？
F. What are black holes made of ？