資料結構與演算法

(4 points) How many of the statements below are known to be true?

• Every problem in NP is polynomial-time reducible to some NP-hard problem.
• Every problem in NP is polynomial-time reducible to some NP-complete problem.
• Every problem in NP is polynomial-time reducible to some problem solvable within polynomial time.
• Every problem in NP is polynomial-time reducible to some problem solvable within exponential time.