Robinson Crusoe Divides His Time Between Catching Fish and Gathering

Robinson Crusoe divides his time between catching fish and gathering fruit.Part of his production possibilities frontier is given in the above table.Mr.Crusoe,while lonesome,is efficient and always stays on his PPF.Mr.Crusoe is consuming 20 pounds of fish.Then he decides to slowly become a vegetarian and decrease his consumption of fish to 9 pounds.This decision means that Mr.Crusoe will

A) incur an opportunity cost of 9 pounds of fruit.
B) incur an opportunity cost of 20 pounds of fish.
C) be able to enjoy a gain of 9 pounds of fruit.
D) incur an opportunity cost of 99 pounds of fruit.
E) incur an opportunity cost of 9 pounds of fish.