Mary is planning to buy a mobile phone that costs $500. a salesperson at the store informs her about four different installment plans that she can choose from.

plan a: a down payment of $50 and monthly payments of $40 for 12 months
plan b: a down payment of $100 and monthly payments of $80 for 6 months
plan c: no down payment and monthly payments of $60 for 12 months
plan d: no down payment and monthly payments of $100 for 6 months

which plan will have the highest finance charge?