The Value of a That Makes $\mathbf { v } = - 2 \mathbf { i } + \mathbf { j } - 4 \mathbf { k }$

Multiple Choice

The value of a that makes $\mathbf { v } = - 2 \mathbf { i } + \mathbf { j } - 4 \mathbf { k }$ and $\mathbf { w } = 3 \mathbf { i } - \mathbf { j } + a \mathbf { k }$ orthogonal is

A) 3
B) $\frac { 7 } { 4 }$
C) 3.
D) $- \frac { 7 } { 4 }$
E) $- \frac { 9 } { 4 }$