In order for a waterfall to form in a stream cutting through a mountainous area, what must be true about the strata/rock underlying the top most layer of the waterfall?

the question isn't complete here so i can't right an answer '_'

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.

the answer would be..

b. sucking

im not intirely sure but i think the anwser is d relative humidity