Keenan walks on the treadmill each morning at a steady rate of 4 miles per hour. His watch keeps track of the total amount of time he has spent exercising. The function D(t) represents the total distance Keenan has walked, in miles, after t hours of walking on the treadmill. What is the domain of D(t) ?

O all real numbers greater than or equal to 0
O all real numbers from 0 to 4
O all whole numbers
O all whole numbers from 0 to 4