"éramos" or "fuimos" for "used to be."

Question

In this question I answered "éramos" instead of "fuimos" since the text referred to "used to be." Why was that wrong? I get that the "completed action in the past" hint confused the situation. Shouldn't the question be clarified, or am I missing something here?
Nosotros ________ muy buenos estudiantes.We used to be very good students.(HINT: Refers to a completed action in the past.)

From the lesson:

"Note that while "fue" is generally translated into "was" in English, "era" can be translated into "was", "used to be" and "would be"."

Answer

Hola Hugo
Sorry about the confusion. I changed both the translation and the hint. The correct answer is "fuimos" so it's no longer saying "we used to..." but "we were" and the hint is now saying that it refers to a completed action in the past.
Gracias y saludos
Inma