A:一定含有甲烷,不含乙烷
B:一定含乙烷,不含甲烷
C:一定是甲烷和乙烯的混合气体
D:一定含甲烷,但不含乙烯
如下图,AB和CD互相____,____是CD的垂线,____是垂足.
When you decide to start a family,you make a/an________to them.You don't turn your back on the people who love you.
下列有关实验操作“先”与“后”的说法中正确的是( )