The first step in mitosis is: a cell splits apart
chromosomes pull away from each other
the nucleus of the cell forms
a second set of chromosomes is produced

A second set of chromosomes is produced, also known as prophase.

Explanation:

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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.