Mary Buys Cell-Phone Services from a Company That Charges $30

Mary buys cell-phone services from a company that charges $30 per month. For that $30 she is allowed 600 minutes of free calls and then pays 10 cents per minute for any calls above 600 minutes. Mary has used 600 minutes this month so far. What is her marginal cost per minute of making additional calls?

A) 25 cents
B) 10 cents
C) 5 cents
D) $0