A:3
B:0.25
C:0.75
D:4
— will you ask for help when you get into trouble? —My parents, I think.
元素符号“N”可以表示( )
老张、老王、老李、老赵是好朋友,一天,他们四人碰面,每两人都握一次手,他们一共握了多少次手?(先画一画,再解答.)