Question: Zulema Castro de Peña was a member of what organization that began to march in 1977 at the Plaza de Mayo in Buenos Aires?
Sentence (from "Mothers of the Plaza de Mayo"): They organized while trying to learn what had happened to their children, and began to march in 1977 at the Plaza de Mayo in Buenos Aires, in front of the Casa Rosada presidential palace, in public defiance of the government's state terrorism intended to silence all opposition.
Relevant evidence: Yes