A:+1
B:+2
C:+3
D:+4
— I'm not going swimming this afternoon.— ______. I have to help my mother do some cleaning.
条件下,X的相对分子质量是( )
西周建立后,今天潍坊一带除齐国外,还有纪国、莱国和杞国等诸侯国,这些诸侯对天子的义务是( )