Two Point Sources Emit Sound Waves of 1

Multiple Choice

Two point sources emit sound waves of 1.0-m wavelength. The sources, 2.0 m apart, as shown below, emit waves which are in phase with each other at the instant of emission. Where, along the line between the sources, are the waves out of phase with each other by π radians?

A) x = 0, 1.0 m, 2.0 m
B) x = 0.50 m, 1.5 m
C) x = 0.50 m, 1.0 m, 1.5 m
D) x = 0.75 m, 1.25 m
E) x = 0.25 m, 0.75 m, 1.25 m, 1.75 m