Suppose Nation a Produces Only Two Goods, Apples and Oranges

Suppose nation A produces only two goods, apples and oranges . If nation A produces only apples, it can make 20 apples per day. If nation A produces only oranges, it can make 15 oranges per day. If the country has a constant production trade- off between apples and oranges, then the opportunity cost of one apple in nation A is:

A) 1.33 oranges.
B) 20 oranges.
C) 15 oranges.
D) .75 oranges.