In this video (https://m.youtube.com/watch?v=KEYYN-a_x6E), the instructor talks about the marginal opportunity cost and supply curve. (Begins talking about it at after 1:50).
He states that the opportunity cost of giving up a quantity of apples (what she is selling) is the opportunity cost.
Is this correct? I thought the opportunity cost is the cost of giving up alternative things. But he plots the points of "giving up an apple' on the marginal opportunity cost curve, even though she is selling apples, so wouldn't that not be the opportunity cost.