21. Let S be an NP-complete problem and Q and R be two other problems not known to be in NP. Q is polynomial-time reducible to S and S is polynomial-time reducible to R. Which of the following statements is true?
(A)R is NP-complete.
(B) Ris NP-hard.
(C) Ois NP-complete.
(D) Q is NP-hard.

1. 題目解析題目給定了三個問題：S...