A:Sure, but I have to visit my grandpa.
B:Not at all. I’ll be glad to.
C:I really can’t agree with you; I prefer science.
D:I’m sorry, but I have to help my sister with her English.
为了保持宇航员在太空舱内的安全,使舱内O2、CO2气体含量的相对稳定,常用NiFe2O4作催化剂将宇航员呼出的CO2转化为O2。NiFe2O4中铁元素的化合价为+3价,则Ni的化合价为 ( )