So lets make a quick split after analyzing the sentence.
I see an extremely helpful split amongst the answer choices..... the use of IS and ARE before "standing waves" is referring to the flow patter, which is singular! Therefore, lets knock out A B and D. See the bold below for more detail.
The Pattern The peaks of a mountain range, acting like rocks in a streambed, produce ripples in the air flowing over them; the resulting flowpattern , with crests and troughs that remain stationary
...
I see an extremely helpful split amongst the answer choices..... the use of IS and ARE before "standing waves" is referring to the flow patter, which is singular! Therefore, lets knock out A B and D. See the bold below for more detail.
The Pattern The peaks of a mountain range, acting like rocks in a streambed, produce ripples in the air flowing over them; the resulting flowpattern , with crests and troughs that remain stationary
...
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