21) A tour bus holds 40 people. It charges $7 per ticket. The function representing the total sales is fx) = 7x, where x represents the number of tickets sold. What is the domain? A) X = all real numbers
B) x = all non-negative integers 280 or belowx = all non-negative integers 40 or below
C) x <=40
D)x = all non-negative integers 40 or below