In the cell cycle, external regulators direct cells to
• A. speed up or slow down the cycle
• B. Remain unchanged
• C. Proceed and then stop the cycle
• D. Grow uncontrollably

the correct answer is:

a population changes in response to a change in the environment.

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.