How much oxygen is contained in the atmosphere?
a. 20%
b. 21%
c. 22%
d. 23%

what is slowed when water is a limiting factor in an environment?
a. amount of evaporation
b. population growth
c. the types of rocks

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 is a) cell division.

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