单项选择题 以下哪种药是亚硝酸盐中毒的解毒药（）

未知题型 GPA is set around 1.8 for each term, so a student should have a GPA of roughly 1.8, with at least 12 credit hours passed if he doesn’t want to be put on probation.

单项选择题 关系模式R(A1，A2，A3，A4，A5)，如果(A1,A2)àA3，(A3,A4)àA5， 则R的候选键为_________。