Which of the Following Functions Are Solutions to the Differential $\frac { d ^ { 2 } y } { d x ^ { 2 } } = - y$

Multiple Choice

Which of the following functions are solutions to the differential equation $\frac { d ^ { 2 } y } { d x ^ { 2 } } = - y$ ? (Mark all correct answers)

A) $y = \sin ( 8 x ) + \cos ( 8 x )$
B) $y = 8 \cos x - \sin x$
C) $y = e ^ { x / 8 }$
D) $y = e ^ { - x / 8 }$