What is the role of the ventral respiratory group (VRG) in the reticular formation of the medulla oblongata?

A: They maintain the basic ventilatory rhythm.
B: Neurons from the VRG stimulate somatic motor neurons to the external intercostal muscles.
C: They sense falling concentrations of O2 or rising levels of CO2.
D: They signal the respiratory center to increase the rate and depth of breathing when the blood is too acidic.