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

[EX Q]: All hairs have a life expectancy of _ to six years.
[EX A]: three

[EX Q]: Willow flycatchers have a lifespan of _ to four years.
[EX A]: three

[EX Q]: Stars are classified into _ main luminosity classes.
[EX A]:
five