In which of these cases are the two wave points considered to be in phase with each other?

A.
One point is on a crest and the other point is on a trough.
B.
One point is at the equilibrium position and the other point is on a trough.
C.
One point is at the equilibrium position and the other point is on a crest.
D.
The two points are both on troughs.

Question

In which of these cases are the two wave points considered to be in phase with each other?

A. 
One point is on a crest and the other point is on a trough.
B. 
One point is at the equilibrium position and the other point is on a trough.
C. 
One point is at the equilibrium position and the other point is on a crest.
D. 
The two points are both on troughs.

Vocaloid · Answer

any ideas? notice how in the "in phase" diagram the corresponding parts of each wave are lined up

|dw:1548533415262:dw|