In response to low blood calcium levels, the parathyroid glands secrete parathyroid hormone, which functions to increase blood calcium levels. this is an example of

Negative feedback loop

Explanation:

When the blood calcium level is low, the receptors of the parathyroid gland sense it and increase the production of cAMP. The increased cAMP levels stimulate the secretion and release of parathyroid hormone from the parathyroid gland into the blood.

The increased parathyroid hormone levels stimulate the number and activity of osteoclasts and speed up the process of bone resorption resulting in the release of calcium ions from bone into the blood to raise the blood calcium levels.

Here, the response (increased blood calcium level) reverse the stimulate and hence, the process is under the regulation of negative feedback loop .

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

the cell cycle is the final answer! : )