The Value of a That Makes $\mathbf { v } = \mathbf { i } + \mathbf { j } - \mathbf { k }$

Multiple Choice

The value of a that makes $\mathbf { v } = \mathbf { i } + \mathbf { j } - \mathbf { k }$ and $\mathbf { w } = 5 \mathbf { i } + a \mathbf { j } + \mathbf { k }$ orthogonal is

A) 8
B) 6
C) 2
D) -2
E) -8