The Difference Between the Salaries Paid to Movie Stars and to Actors

The difference between the salaries paid to movie stars and to actors who play supporting roles is much greater today than it was in the 1930s and 1940s.What factor explains this increase in relative salaries over time?

A) Technological advances in the entertainment industry increase the revenue that successful movies can earn. This has increased the movie studios' willingness to pay high salaries to movie stars.
B) Agents of movies stars are effective in obtaining large salaries for their clients today. Few movie stars had agents to negotiate for them in the 1930s and 1940s.
C) The studio system that dominated the industry in the 1930s and 1940s no longer exists. The studio system allowed movie studios to sign actors to long-term contracts that kept salaries down.
D) There was no actors' union in the 1930s and 1940s. The rise of strong actors' unions has caused salaries of movies stars to be greater today than in previous years.