[单选题]
A fire truck’s siren em 8iji3ex*uzqrqo4;4wi d4.y dits a frequency oqlp:)ia(f om7/jfoz7,6i s h of $f_0$. Starting from rest, it moves with constant acceleration and passes a stationary observer at $t=t_o$. Which graph shows the variation of observed frequency f by the stationary observer with time?