A shale oil deposit is found in a farming area. For the next ten years, the following estimates are given. If the land were used only for farming, it would generate income after tax of $\$10$ billion. If the land were used only for shale oil extraction, it would generate income after tax of $\$40$ billion. The government would collect $\$5$ billion in taxes from farming and $\$10$ billion in taxes from shale oil extraction. What is the opportunity cost of using the land only for farming?
- A$\$25$ billion
- B$\$30$ billion
- C$\$35$ billion
- D$\$45$ billion