The cost opportunity from B to D is 15 surfboards, while the cost opportunity from D to A is 6 motorcycles.
In economics, opportunity cost refers to what is lost if one alternative is chosen over another. For example, if a country decides to manufacture only cars they are losing the opportunity to manufacture other products such as cellphones.
If you choose Combo B instead of D you lose the opportunity to manufacture 15 more surfboards
If you choose Combo A instead of D, you lose the opportunity to manufacture 6 motorcycles
Points F and G imply that if you manufacture 3 motorcycles you can only manufacture 21 surfboards (F), and if you manufacture 5 motorcycles the production of surfboards increases by 3 (24 surfboards).
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