what would happen to the potential difference if the connecting leads of the voltmeter were swapped around?

A) acceleration. this seems to be the one that makes sense.

Independent variable is something that is changed in a experiment. 

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.