Does anyone have the answer key for the Potential energy shelves gizmo?

It really doesn't matter for any of the questions that you've listed.

Answer4 million;

The correct option is this: the sound waves from the two trombones have slightly different frequencies.from the question given above, we are told that the loudness of the sounds were increasing and decreasing, this is because the frequencies of the two sounds are different. the means that the number of waves that pass a fixed point in a given amount of time differ for both sounds.