Fully Constructive Interference Between Two Sinusoidal Waves of the Same $\degree$

Multiple Choice

Fully constructive interference between two sinusoidal waves of the same frequency occurs only if they:

A) travel in opposite directions and are in phase
B) travel in opposite directions and are 180 $\degree$ out of phase
C) travel in the same direction and are in phase
D) travel in the same direction and are 180 $\degree$ out of phase
E) travel in the same direction and are 90 $\degree$ out of phase