You'd better ask a ______, such as your aunt or sister, to look after your children while you are away.
A:classmate
B:neighbor
C:relative
密闭容器中有甲、乙两种物质各10g,加热一段时间后测得容器中各物质的质量如表所示
物质
甲
乙
丙
丁
反应后质量/g
1.6
X
0.9
2.2
下列选项正确的是( )
Once ____ ____ ____ ____ ____ ____, nothing can change it.