Fill up the blanks in the following paragraph by selecting the correct option.
Human beings have a significant ability to maintain and moderate the respiratory rhythm, to suit the demands of the body tissues. This is done by the neural system. A specialised centre present in the medulla region of the brain called ___(i)___ is primarily responsible for this regulation. Another centre present in the pons region of the brain called ___(ii)___ can moderate the functions of the respiratory rhythm centre. Neural signal from this centre can reduce the duration of ___(iii)___ and thereby alter the respiratory rate. A __(iv)__ is situated adjacent to the rhythm centre which is highly sensitive to CO2 and hydrogen ions.