The slope would represent the cost per minute, since m is the length of the call in minutes. Logically, D) Cost per minute, is the only one that would work. The connection cost would be just added in, and you wouldn't multiply the cost of having a phone line by how many minutes you're on the phone. The length of the call is already there, it's m, so that wouldn't work either. Therefore, D, cost per minute, is the logical answer. The slope in the equation represents D, cost per minute.
