A long time ago, a tribe of natives on the mythical continent of Marwinian could produce two commodities to eat. They could harvest fish from the sea and they could grow grain from the arable rainforest. The table and graph below both show the maximum annual output combinations of fish and grains that could be produced by the natives of Marwinian.

What would have been the opportunity cost of increasing the annual output of grain from 0 bushels up to 300 bushels (starting from the absolute PPF position where there was no grain production and all fishing activity)?