The total revenue for a bluetooth speaker (in dollars) is given by R(x) = 35x -0.01x^2 where x is the number of units sold. What is the average rate of change in revenue as the number of bluetooth speakers sold increases from 20 to 40 units? (1 point)
O 34.40 units per dollar
O $0.30 per unit
O -$34.40 per unit
O $34.40 per unit