Concept:International co-operation among nations is classified by the number of countries involved in the relationship.
Explanation:Bilateral co-operation means a relationship or agreement between exactly two countries.
Multilateral co-operation means a relationship or agreement involving three or more countries.
These two categories describe the main types of co-operation that exist among nations.
Option A is incorrect because "cultural" is an area of co-operation, not a main type.
Option C is incorrect because "economic" and "technical" are also areas of co-operation, not structural types.
Option D is incorrect because "educational" is likewise an area, and it does not pair with "multilateral" as one of the two main types.
Therefore, the correct classification is based on the number of participating nations.
Answer:B. bilateral and multilateral.