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

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

D  

these are 'in phase'  

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The answer is d

Explanation:

Ik because I have the same question put d and got it right

b. is the answer hoped i