In recent years, manufacturing firms in the U.S. have laid off workers. The best explanation of this trend is that:

A) firms have been trying to weaken unions by reducing their reliance on labor.
B) workers in the U.S. have become less productive and are no longer able to compete in the global economy.
C) firms in many industries have become so efficient that they need fewer workers.
D) the economy has been in a prolonged depression that has lasted more than a decade.