A cell phone plan charges $15 per month for unlimited call and text messages, however each gigabyte (GB) of data is charged at a rate of $7 per GB. Assume that fractional values of a GB are prorated according to this rate. If a user uses more than 12 GB of data during their monthly billing cycle, then they are not charged more than the cost for the first 12 GB. Write a piecewise-defined function that describes the amount a cell phone user will pay, C, as a function of the number of GB of data they use, d, over the course of a month.