[单选题]
The frequency of a t4yfy0ugg cyr;p0)a,i raxsry05b7rx ef)ka :x+in horn is 500 Hz. Assume the speed of sound in air is 340 m/s. What is the frequency heard by an observer if the observer is approaching the train with rxb :x7 kxa+r)fe0y s5a speed of 30.0 m/s?
A. $456 \; Hz$
B. $500 \; Hz$
C. $544 \; Hz$
D. none of the above