Let $\mathbf { r } ( t ) = \cos ( 2 t ) \mathbf { i } + \sin ( 2 t ) \mathbf { j } + 3 t \mathbf { k }$

Multiple Choice

Let $\mathbf { r } ( t ) = \cos ( 2 t ) \mathbf { i } + \sin ( 2 t ) \mathbf { j } + 3 t \mathbf { k }$ Then the unit normal vector $\mathbf { n } \left( \frac { \pi } { 4 } \right)$ is

A) i
B) $- \frac { 1 } { \sqrt { 5 } } \mathbf { i } + \frac { 4 } { \sqrt { 5 } } \mathbf { k }$
C) $\frac { 2 } { \sqrt { 29 } } \mathbf { j } + \frac { 5 } { \sqrt { 29 } } \mathbf { k }$
D) $- \mathbf { j }$
E) k