How to calculate the marginal cost of the following gasoline price?

Question

A driver wants to buy gasoline and have her oil checked,too. The price of gasoline at the pump is 2 dollars per litre. The pricing for the oil check varies: it's 1 dollar when she buys 49 litres and 0.75 dollars when she buys 50 litres. What is the marginal cost of the 50th litre of gasoline.

In my opinion, since the price equals marginal cost, isn't marginal cost 2 dollars?

Answer 1

The price of gasoline is the cost function $c (q)=2q$ where $ q $ is the quantity; so the marginal cost $c'(q)=2$.

Answer 2

The marginal cost is always defined as the first derivative of the cost function.

So MC(c) = C'(c) = (2c)' = 2. Hence MC(50) = 2. In fact, the marginal cost is constant.