18. 電腦科學領域中，電腦科學家普遍認為P  NP，請問下列有關P、NP、NP-Complete與NP-Hard的敘述，何者不正確？ (A)If a problem X is in P, X is also in NP. (B)If a problem X is in NP-Complete, X is also in NP. (C)If a problem X is in NP-Complete, X is also in NP-Hard. (D)If a problem X is in NP, X is also in NP-Complete.