Two in-phase speakers (S₁) and (S₂) emit sounds with a frequency of 136 Hz. Both speakers are aligned along the (x)-axis. A person starts at the origin and slowly walks along the (t)-axis.

a. The person hears no sound.
b. The person hears sound with increasing frequency.
c. The person hears sound with decreasing frequency.
d. The person hears sound with constant frequency.