单选题：Which of the following is **TRUE** about NP-Complete and NP-Hard
Which of the following is **TRUE** about NP-Complete and NP-Hard problems? @[D](2)
A. If we want to prove that a problem X is NP-Hard, we take a known NP-Hard problem Y and reduce Y to X in polynomial time.
B. The first problem that was proved to be NP-complete was the circuit satisfiability problem.
C. NP-complete is a subset of NP-Hard
D. All of the other three.
A.If we want to prove that a problem X is NP-Hard, we take a known NP-Hard problem Y and reduce Y to X in polynomial time.
B.The first problem that was proved to be NP-complete was the circuit satisfiability problem.
C.NP-complete is a subset of NP-Hard
D.All of the other three.
答案：D
A. If we want to prove that a problem X is NP-Hard, we take a known NP-Hard problem Y and reduce Y to X in polynomial time.
B. The first problem that was proved to be NP-complete was the circuit satisfiability problem.
C. NP-complete is a subset of NP-Hard
D. All of the other three.
A.If we want to prove that a problem X is NP-Hard, we take a known NP-Hard problem Y and reduce Y to X in polynomial time.
B.The first problem that was proved to be NP-complete was the circuit satisfiability problem.
C.NP-complete is a subset of NP-Hard
D.All of the other three.
答案：D