Use Vectors to Find the Measure of the Angle at Vertex

Multiple Choice

Use vectors to find the measure of the angle at vertex B of triangle ABC, when $A = ( 5,1 )$ , $B = ( - 5,1 )$ , and $C = ( - 3 , - 5 )$ .Round answer to two decimal places.

A) 69.48°
B) 71.57°
C) 72.91°
D) 70.38°
E) 74.09°