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Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question.

The equation c = ar + b tells us that the cost is increases by $a when the revenue is increases by $1.
It follows that the cost is increases by $10a when the revenue is increases by $10. So, the question is asking for the value of a.

The answer is A.
Answer: A