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