An economy produces only agricultural products (x) and manufactured goods (y). A quarter of the land in the country is arid and not suitable for agricultural products. Therefore, if half of the arid land is not used at all:
(a) The economy could be productively efficient when the opportunity cost of x (agricultural products) is infinite;
(b) The economy could be productively efficient when the opportunity cost of y (manufactured goods) is zero;
(c) The economy could be productively inefficient with infinite opportunity cost for the production of x (agricultural products);
(d) The economy could be productively inefficient with infinite opportunity cost for the production of y (manufactured goods).