Our offer _____ and remains good untill 12:00 A.M. July 12, 2003

Our offer _____ and remains good untill 12:00 A.M. July 12, 2003, Beijing Time.
A、are firm offers
B、are firm offer
C、is firm offer
D、is a firm offer
【正确答案】：D
【题目解析】：offer是单数名词，谓语动词用单数。