Answer: All of the above.
Step-by-step explanation:
Countries engage in international trade because they've different resources. Therefore, when a country doesn't have a particular resource, it engaged in trade with the one that has.
Also, some countries are better than others at producing goods and services. This is where absolute and comparative advantage comes in. So, when two countries trade, the countries will engage in trade where they've comparative advantage.
Also, some countries have a climate more suited to the production of certain types of products. For example, certain crops grow in some countries while it doesn't grow in others.
Therefore, the answer is all of the above.