People who have done a lot of endurance training can have a resting heart rate of 50 beats per minutecompared to the normal resting heart rate of 70-80 beats per minute. This decreased heart rate in trainedindividuals is the partly result of:

A) endorphins released from the brain which have an inhibitory effect on heart rate.

B) parasympathetic activity becoming more active and sympathetic activity becoming less active at rest.

C) decreases in both sympathetic and parasympathetic activity.

D) sympathetic activity becoming more active and parasympathetic activity becoming less active at rest.

E) increases in both sympathetic and parasympathetic activity