Define control group on your own words

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.

a=c and then d= c because of the band so you get ac/dc and you add them together you get sound waves

c is the answer. hope this