Over a Ten Year Period, the Monthly Charge for Cellular

Over a ten year period, the monthly charge for cellular phone service decreased from $120 per month to $30 per month. At the same time, the number of subscribers increased from less than 10 million to more than 75 million. Which of the following provides the best explanation for these changes?

A) An increase in consumer income over this ten year period
B) A reduction in the price of residential phone service, a substitute for cellular phone service
C) An increase in the wages of workers in the cellular phone industry
D) Technological improvements that reduced the cost of supplying cellular phone service