Two speakers both emit sound of frequency 320 Hz, and are in phase. A receiver sits 2.3 m from one speaker, and 2.9 m from the other. What is the phase difference between the two sounds detected by the receiver?

a. 32.3 λλ
b. 0.60 λλ
c. 0.089 λλ
d. 0.29 λλ
e. 0.56 λλ