A Car with Its Horn Sounding Approaches a Group of Students

A car with its horn sounding approaches a group of students. Assume the car's horn produces sound waves with a constant frequency. Which of the following statements best explains why the students hear a higher pitch as the car approaches than when it is stopped?

A) The sound waves increase in speed as the car approaches the students.
B) The sound waves decrease in speed as the car approaches the students.
C) The sound waves are heard at a lower frequency as the car approaches the students.
D) The sound waves are heard at a higher frequency as the car approaches the students.