Karlena is on an old cell phone plan. The cost of a call depends on the number of minutes the phone call lasts. She can use the expression $0.10m+3.500.10m+3.50 to find the cost of a call. If m is the number of minutes, which of the following best describes $3.50?
The total cost of a call
A fee that is charged per call
The cost per minute
How long the call lasted