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. not all goods and services are traded internationally, production of most goods involves constant opportunity costs, and tastes for products are remarkably uniform.
d. all goods and services are traded internationally, production of most goods involves increasing opportunity costs, and tastes for products are remarkably uniform.