Agnes can produce either 1 unit of X or 1 unity of Y in an hour, while Brenda can produce either 2 units of X or 4 units of Y in an hour. The opportunity cost of producing a unit of X is :

A) 1 unit of Y for Agnes and 2 units of Y for Brenda

B) 1 unit of Y for Agnes and 1/2 unit of Y for Brenda

C) 1 hour for Agnes and 1/2 hour for Brenda

D) 1 hour for Agnes and 2 hours for Brenda