The cells that respond to changes in blood gas concentration and generate the respiratory rhythm are found in several nuclei in the

The answer is; medulla oblongata and pons.  

In this region of the brain, there are chemoreceptors that can detect changes in the pH of the blood. These are called central chemoreceptor. There are also chemoreceptors in the arteries called peripheral chemoreceptors. When these chemoreceptors detect a drop in the blood PH (due to increased dissolved carbon dioxide in the blood), the depth and rate of breathing is increased in the lungs.  

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

the answer is a) cell division.

hope this !