Over a ten year period, the monthly charge for cellular phone service decreased from $80 per month to $30 per month. At the same time, the number of subscribers increased from less than 5 million to more than 125 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 population and standard of living over this ten year period
d) Technological improvements that reduced the cost of supplying cellular phone service