Here's why:
* Subject-Verb Agreement: The sentence is talking about Alma and Sonia doing the homework. "Alma y Sonia" is plural, so the verb needs to be in the plural form as well.
* Context: The sentence implies that Alma and Sonia completed the homework. The verb "hicieron" means "they did" in the past tense, which fits the context.
The complete sentence is:
Dijimos que Alma y Sonia hicieron toda la tarea anoche antes de acostarse.
This translates to: "We said that Alma and Sonia did all the homework last night before going to bed."