A because one time fees for a phone upgrade or initial phone are just one time. Look at the monthly fees instead would you rather pay more each month or less each month? If answer is less A is the best choice even though in the beginning you need to pay more.
For lines to be parallels the slopes must be equal, for the lines to be perpendicular the lines have to be inverse reciprocals meaning if you have a slope of 1/2X then the perpendicular line must be -2X