which one of the following concerning unit vectors is true?

a$)$ unit vectore are predominantly used in mathematics, but seldom used in physics

b$)$ a unit vector aways points in the direction of motion

c$)$ the magnitude of a unit vector sometimes equals zero

d$)$ a unit vector depends on the units of measurment used and is a method of tracking the units throughout a calculation

e$)$ the magnitude of a unit vector is always equal to $1$