If Germany used all its resources to produce bread, suppose it could produce 40 million loaves of bread. If it used all its resources to produce milk, it could produce 20 million gallons of milk. Then Germany's opportunity cost of producing a ______ is ______.

a) loaf of bread; 2 gallons of milk

b) loaf of bread; 1/2 a gallon of milk

c) gallon of milk; 1/2 a loaf of bread

d) gallon of milk; 1 loaf of bread