[单选题]
A light source approaches a stationary receiver at g2os6ly sj( z0d p0ub4kbn.i/ adcunc6. /rs f6i v+g+qn increasing speed. The frequency of the emitted light is constant. Which graph shows how the observed frequency f bi./ cnq 6fvu+d sr6cg+y receiver varies with the speed v of the light source?