A service provider charges c dollars for the first 50 hours of service used per month and 40 cents for each 30 minutes in excess of 50 hours used during the month. If x is an integer greater than 50, which of the following expressions gives this service provider’s charge, in dollars, for a month in which x hours of service were used?

A. c + 0.40x
B. c + 0.80x
C. c + 0.40(x – 50)
D. c + 0.80(x – 50)
E. c + 0.40(2x – 50)

D