Write the Rate as a Unit Rate $\mathrm { mi } / \mathrm { hr }$

Write the rate as a unit rate.
-126 miles in 6 hours

A) 132 mi/hr $\mathrm { mi } / \mathrm { hr }$
B) 21 mi/hr $\mathrm { mi } / \mathrm { hr }$
C) 21 mi
D) 756 mi