Let $\mathbf { r } ( t ) = \sin ( 2 t ) \mathbf { i } - \cos ( 2 t ) \mathbf { j } - 4 t \mathbf { k }$

Multiple Choice

Let $\mathbf { r } ( t ) = \sin ( 2 t ) \mathbf { i } - \cos ( 2 t ) \mathbf { j } - 4 t \mathbf { k }$ Then $\mathbf { r } ^ { \prime } \left( \frac { \pi } { 4 } \right)$ is

A) $2 \mathbf { j } - 4 \mathbf { k }$
B) $2 \mathbf { j } + 4 \mathbf { k }$
C) $- 2 \mathbf { j } - 4 \mathbf { k }$
D) $- 2 \mathbf { j } + 4 \mathbf { k }$
E) $2 \mathbf { i } - 4 \mathbf { k }$