We do not see complete specialization in the real world because
A. not all goods and services are traded internationally, production of most goods involves increasing opportunity costs, and tastes for products differ.
B. not all goods and services are traded internationally, production of most goods involves decreasing opportunity costs, and tastes for products differ.
C. all goods and services are traded internationally, production of most goods involves increasing opportunity costs, and tastes for products are remarkably uniform.
D. not all goods and services are traded internationally, production of most goods involves constant opportunity costs, and tastes for products are remarkably uniform.

The concept of complete specialization in trade is not applicable in the real world. A country can have partial specialization but not complete specialization. This is because not all the goods are produced for trading in foreign markets. Most of the goods are consumed in the domestic market.

The taste and preferences of consumers in different markets differ. Also as a country goes on increasing the quantity produced of a good, the opportunity cost of giving up its second-best alternative goes on increasing. So the countries will not reach complete specialization.