4) These 3 things are essentially the same thing (with slightly different angles).

When they have slightly different frequencies.  This is because they will fall into and out of phase, leading to constructive and then destructive interference.  This change in interference causes a pulsating nature to the volume that is called beating

Explanation:

"girls"

explanation:

a subject of a sentence is a noun or pronoun that shows who/what is performing the action (or verb) of the sentence.

in this case, the sentence states that the girls see. the girls are the ones who are performing the action, so they are the subject.

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