Robert decides to go for a run when he gets home from school. as he begins running, his breathing rate starts to increase. what are the mechanisms that cause his breathing rate to increase? a. when carbon dioxide levels in the blood decrease, the respiratory center sends signals to the alveoli to increase the volume that they expand. b. when oxygen levels in the blood decrease, the respiratory center sends signals to the diaphragm to increase the rate at which it contracts and relaxes. c. the respiratory control center senses the need for more oxygen, which causes the trachea to push and pull air more forcefully. d. the diaphragm senses the need for more oxygen, which causes it to expand and contract more quickly in order to inflate and deflate the lungs.