[单选题]
A light source approaches a stationary recenq pf p5o/,* h46bua,img 8rm3ywp4r5ot* rmbiver at an increasing speed. Tmeh8k bt8ro42wd;i pd9xgl59ru rw3*he frequency of the emitted light is ckeu8w4lh2b; xo8d *p3m g9idr9 rr5w tonstant. Which graph shows how the observed frequency f by receiver varies with the speed v of the light source?