Teacher: In this task, we ask you to write an answer to a question about when an event happened. For example, "going to school" usually happens during the day (not at 2 A.M). Note that a lot of the questions could have more than one correct answer. We only need a single most-likely answer. Please try to keep your "answer" as simple as possible. Concise and simple "answer" is preferred over those complex and verbose ones.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Sentence: Jack played basketball after school, after which he was very tired. 
Question: When did Jack play basketball?
Solution: 4 PM.
Reason: This event likely happened between 3PM to 8PM. This is a good answer.

Now, solve this instance: Sentence: A floodplain is an area where a thick layer of rich soil is left behind as the floodwater recedes. 
Question: When did it flood?
Student:
yesterday.