Pretty much the more activity you do, or the more intense it is, the faster your heartrate will be. Your heart needs blood, and when you're exercising the blood is pumping through your body faster.
Answer: Exercise causes the heart to pump blood into the circulation more efficiently as a result of more forceful and efficient myocardial contractions, increased perfusion of tissues and organs with blood, and increased oxygen delivery. Aerobic exercise trains the heart to become more efficient.
"C. Organisms' characteristics are carried
<span> on DNA.</span>" would be the best fit, although this still isn't really the core idea behind natural selection, which is that animals survive based on how "fit" they are for their environment.
