Deep breathing rises the $O _2$ level of the blood. By excess formation of oxyhaemoglobin the acidity of blood increases. To control it, more of free $CO _2$ changes into bicarbonates and the free $CO _2$ level in the blood decreases. So due to lack of stimulant for inspiratory centre cessation of breathing takes place, after some time $O _2$ is consumed by the tissues and rise in $CO _2$ stimulates breathing again. So, the correct answer is 'Both too much O$ _2$ and very little CO$ _2$ in the blood'.

When the concentration of carbon dioxide increases in blood that results in decreasing the affinity of hemoglobin towards oxygen or amount of oxygen is less in blood. To compensate the oxygen amount breathing goes faster and deeper and is known as hyperventilation which is controlled by the medullary respiratory center.

• Regulation of Respiration is done by the nervous mechanism.
  
• A specialised centre present in the medulla region of the brain called the respiratory rhythm centre is primarily responsible for this regulation.
  
• Another centre is present in the Pons region in the brain called pneumotaxis centre can moderate the function of Respiratory Rhythm Centre.
  

• The respiratory center is located in the medulla oblongata and pons, in the brainstem.
• The respiratory center is made up of three major respiratory groups of neurons, two in the medulla and one in the pons. It is responsible for generating and maintaining the rhythm of respiration.
• Hence Respiratory rhythm center is present Medulla and pons.
• So, the correct answer is 'Medulla and Pons.

Respiratory centres generate rhythmic pattern of breathing and are present in brain stem. The medullary respiratory centres constitute the primary respiratory centre and include two aggregates of neuronal cell bodies namely dorsal respiratory group and ventral respiratory group. They serve to provide output to respiratory muscles. Pontine respiratory centres are present in higher in brain stem in pons and are namely pneumotaxic centre and apneustic centre. Pontine respiratory centres modify the output from medullary centres.

•  Cellular respiration is the process by which cells convert glucose into carbon dioxide and water. 
  
• Cellular respiration takes place in three steps: glycolysis, the citric acid cycle and oxidative phosphorylation.
• The main role of enzymes during the respiration reaction is to assist in transferring electrons from one molecule to another. e.g. In glycolysis, the enzyme dehydrogenase is used. Hence, the steps of respiration are controlled by enzymes.

The medulla oblongata is the lower half of the brainstem, which is continuous with the spinal cord, the upper half being the pons. The medulla contains the cardiac, respiratory, vomiting and vasomotor centres and therefore deals with the autonomic (involuntary) functions of breathing, heart rate, and blood pressure.

(e) Inspiration is the result of muscular contraction. The diaphragm and external intercostal muscle contracts simultaneously. The lateral thoracic wall moves outward and upward.

Respiratory volumes and capacities Volume of air

Rate of breathing is mainly controlled by $CO _2$ level in the blood. Excess $CO _2$ decrease the pH of the blood and it mainly stimulate the respiratory centre of the brain (i.e., Medulla oblongata) and increase the inspiratory and expiratory signals to the respiratory muscles.

The rate of breathing is controlled mainly by CO$ _2$ level in blood. The partial pressure of carbon dioxide (PCO$ _2$) controls the respiratory rate. The respiratory centre measures the levels of carbon dioxide, oxygen and acidosis in the arterial blood and sends message accordingly. If the carbon dioxide levels in the blood increases, the rate and depth of breathing increases.

An increased concentration of carbon dioxide is the strongest stimulus to breathe more deeply and more frequently. When the carbon dioxide concentration in the blood is low, the brain decreases the frequency and depth of breath.

A common laboratory technique used to make many copies (millions or billions) of a particular region of DNA is called Polymerase Chain Reaction(PCR). It requires a primer, DNA helicases, DNA polymerases, etc.,

So, the correct option is ‘ Nucleotide precursors’.

Receptors associated with aortic arch and carotid artery can recognise a change in $CO _{2}$ and $H^+$ concentration and send necessary signals to the rhythm centre (medulla oblongata) that controls the rate of breathing.