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) 15 oranges.
C) 20 oranges.
D) .75 oranges.