People Have a Tendency to Allow Their Minds to Wander

People have a tendency to allow their minds to wander while attending a conference or a business seminar because:

A) much of what people say is expressed by nonverbal.
B) people are in the habit of indulging in selective listening.
C) most humans have a capacity for listening at a speed that far exceeds the ability of the fastest speaker.
D) from childhood, most people are trained to listen with less tolerance for distractions and more feedback to the speaker.