What factors, other than exercise, would increase the heartbeat rate?

Emotions and anxiety can raise your heart rate, Body Temperature: If you become too hot or too cold your body senses a thermal stress load, The terrain, Wind, Dehydration, Diminishing glycogen stores — your muscles primary fuel source.

Explanation:

a. decomposes use carbon from dead organisms as fuel for cellular respiration, returning carbon dioxide back into the atmosphere.

the answer for this question would be d. glaciers