Answer:
The correct answer is knew.
Step-by-step explanation:
Action verbs are all those verbs that express in their meaning the actions carried out or suffered by a subject.
This subject can be a person, animal, object or any entity that performs the action of the verb.
An action refers to something that is carried out or done.
In this example, the main action is carried out by "us" (We), so that makes the action verb "knew".