A:①②
B:②③
C:③④
D:②④
_______ isn't afraid of swimming in the sea. ( )
按图示的规律搭下去,则所用火柴棒数an 与所搭三角形的个数n之间的关系式可以是____.
①卢沟桥事变 ②西安事变 ③九一八事变 ④南京大屠杀