Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Problem: if the price of petrol increases by 40, by how much must a user cut down his consumption so that his expenditure on petrol remains constant ?
Options: a. 25 %, b. 20 %, c. 16.67 %, d. 28.6 %, e. none of these
d