B1
used at the beginning of a story to mean a long time ago
Dictionary examples:
Once upon a time, a lazy student invented a homework-doing robot. (0.0)
Once upon a time, a forgetful king lost his entire kingdom. (0.0)
Once upon a time, a teacher gave homework to her students' pets. (0.0)
Once upon a time there was a beautiful princess. (27.6)