During four successive years a car owner filled the car's tank with petrol purchased at the respective costs of 0.80. 0.90, 1.05 and 1.25 per litre. Assuming the car owner purchased 1000 litres each year, the average cost of petrol over the four-year period is A 1 per litre B 2 per litre C 3 per litre D 4 per litre