The monthly electricity bill raised by the municipal corporation consists of two parts-a fixed charge for providing the service and a variable charge which is directly proportional to the number of watts of power consumed. An amount of Rs. 500 is charged for consuming 125 watts in a particular month . The of 500 watts is `21 : 25` . How much is the fixed charge per month?
A. Rs. 125
B. Rs. 200
C. Rs.250
D. Rs. 500