Which of the following, if true, most strengthens the author's argument concerning the debt of the acto to the theater traditions of other periods and regions? A. Many popular forms of theater rely heavily on improvisation. B. Plays resembling the acto in structure were written in the 1970's by West African playwrights who are interested in dramatizing the richness of their own cultures. C. The use of masks has, at one time or another, been characteristic of the theater traditions of almost all cultures, even those most isolated from outside influences. D. During a strike, it is common for union members to present musical skits dramatizing the values of solidarity and resistance. E. Before 1965 Luis Valdez had attended many performances of traditional Mexican theater groups touring the western United States.