Which of the following statements regarding the heckscher-ohlin model and Ricardian trade theory is TRUE?

a. Both the Heckscher-Ohlin and Ricardian models are current, relevant, and applicable in explaining world trade patterns

b. The Ricardian model is an excellent simplified introduction to world trade patterns, but the Heckscher-Ohlin model is more relevant in regards to explaining empirical evidence

c. The Ricardian model is useless in explaining trade patterns, and the H-O theory, though applicable, is not sufficient for explaining real world trade patterns

d. The Ricardian model is extremely useful in analyzing trade patterns, but the H-O model is only useful in theoretical applications.