You are standing some distance away from two speakers that are arranged one in front of the other and in front of you. They emit identical sound waves. As the speaker farther away from you moves 40 cm toward you, the sound you hear changes from maximum loudness (constructive interference) to no sound at all (destructive interference).

How much farther towards you will the speaker have to be moved so that you hear maximum loudness once again?

Can you please show how (C) is the correct answer. Thank You!

A. 80 cm

B. 20 cm

C. 40 cm

D. 10 cm