A bank offers loans with simple interest rates for those who qualify. If the principal is the same for each, which two plans charge the least interest?
A.Plan A: a 20% interest rate to be paid off in 5 years
B.Plan B: a 10% interest rate to be paid off in 8 years
C.Plan C: a 5% interest rate to be paid off in 15 years
D.Plan D: a 25% interest rate to be paid off in 18 months.