What is a trough how is it different than crest

A trough is different from a crest in that the crest is the highest part of a sound wave, while the trough is the lowest part of the wave.

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the apollo program, also known as project apollo, was the third united states human spaceflight program carried out by the national aeronautics and space administration (nasa), which succeeded in landing the first humans on the moon from 1969 to 1972.