A manufacturer of jet engine harnesses has weekly production costs of C = 0.25x2 - 10x + 800 where C is the total cost (in dollars) and x is the number of units produced. What is the average rate of change in the cost per unit as the manufacturer increases the weekly production from 500 to 600 units?
A. $265 per unit
B. $274 per unit
C. $583 per unit
D. $848 per unit