How Does the NAFTA Seem to Increase the International Competitiveness

How does the NAFTA seem to increase the international competitiveness of U.S. and Canadian firms?

A) It gives them access to scarce natural resource hitherto unavailable.
B) It increases tariff barriers making the North American market less attractive to Asian companies.
C) It allows them to take advantage of lower labor costs in Mexico.
D) It gives them production bases in South American markets like Brazil and Argentina.