Opportunity cost — practice question

A farmer cuts the area used for growing wheat from $80$ hectares to $60$ hectares and raises the area used for growing potatoes from $80$ to $100$ hectares. What is the opportunity cost of this change?

• AThe output from $20$ hectares used for growing potatoes.
• BThe output from $20$ hectares used for growing wheat.
• CThe output from $60$ hectares used for growing wheat.
• DThe output from $80$ hectares used for growing potatoes.