In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.

[Q]: Healthy people can donate blood as often as once every _ weeks.
[A]: eight

[Q]: Imago is a group of _ young musicians whose love for music is full of passion.
[A]: four

[Q]: Most Edwardian castles have _ concentric rings of walls and towers.
[A]:
three