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

Multiple Choice

Let $\mathbf { r } ( t ) = \cos ( t ) \mathbf { i } - \sin ( t ) \mathbf { j } + \pi t \mathbf { k }$ Then the unit normal vector $\mathbf { r } ( t ) = \cos ( t ) \mathbf { i } - \sin ( t ) \mathbf { j } + \pi t \mathbf { k }$ is

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