A:0~0.3s和0.3~0.6s
B:0.6~0.9s和0.9~1.2
C:0~0.3s和0.9~1.2s
D:0.3~0.6s和0.9~1.2
已知指数函数y=f(x)的图象过点(2,4),若f(m)=8,则m=____