Petrol is sold . . . . . . . . the liter. A. by B. on C. at D. from

The correct answer is C. at.

Petrol is sold at a price per liter. This means that the price of petrol is determined by the amount of petrol that is purchased. For example, if the price of petrol is $2.00 per liter, then a customer who purchases 5 liters of petrol will pay $10.00.

The other options are incorrect because they do not accurately describe how petrol is sold. Option A, by, is incorrect because it suggests that petrol is sold by a specific person or entity. Option B, on, is incorrect because it suggests that petrol is sold on a specific day or date. Option D, from, is incorrect because it suggests that petrol is sold from a specific location.