What is the Ricardian model of comparative advantage based on?

The fact that countries are different and can benefit from trading with each other by specializing in what they do relatively well.
The need to achieve economies of scale in production by producing only a limited range of goods.
The idea that countries should only trade with each other if they have similar levels of productivity.
a) Only I is correct.
b) Only II is correct.
c) Only III is correct.
d) I and II are correct.
e) I and III are correct.