In any experiment there must be at least 2?

If you're asking for what 2 things must be present for an experiment to happen is "Independent Variable and Dependent Variable"

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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