Question

eyebrow hairs are always shorter than hairs on your head because

hormones in the eyebrow follicle switch the growth off after it
has reached a predetermined length

eyebrow follicles are only active for a few months

they grow much slower

the vascular supply of the eyebrow follicle is one - tenth that of
the head hair follicle

Explanation:

Hair growth is regulated by the hair follicle cycle. The anagen (growth) phase for eyebrow follicles is shorter (only a few months) compared to head hair follicles which have a longer anagen phase. Hormones don't "switch off" at a predetermined length in a simple on - off manner as described in the first option. Eyebrow hair growth rate is not the main reason for their shorter length compared to head hair. Vascular supply differences are not the key factor for the length difference.

Answer:

eyebrow follicles are only active for a few months