Which formula should you use to find the distance between the two points (xl, yl, zl) and (x2, y2, z2) in 3D space?

Which formula should you use to find the distance between the two points (xl, yl, zl) and (x2,
y2, z2) in 3D space?

A.
Option A

B.
Option B

C.
Option C

D.
Option D

Explanation:
http://freespace.virgin.net/hugo.elias/routines/r_dist.htm