Eyebrow hairs are always shorter than hairs on your head because a. hormones in the eyebrow follicle switch the growth off after it has reached a predetermined length b. eyebrow follicles are only active for a few months c. the vascular supply of the eyebrow follicle is one-tenth that of the head hair follicle d. they grow much slower

The answer is B. Hairs have different lengths due to variations in the activity of their respective hair follicles. Hair has growth and resting phases. The hairs on the head have longer growth phases compared to those of the eyebrows. Therefore, they grow longer before they start falling out in the resting phase. Those of the eyebrows get to the resting phase quicker hence are shorter

I believe the answer is: B.  eyebrow follicles are only active for a few months
Follicles
Follicle is a hormone that could influence the growth of hair in our body.
The follicles of the hair on our head could active for years depending on the nutrition that it gets.
Eyebrow follicles on the other hand, only active between 2-4 months on average.