For What Values Of $k$ Does the Function $y = \cos k t$

Multiple Choice

For what values of $k$ does the function $y = \cos k t$ satisfy the differential equation $49 y ^ { \prime \prime } = - 81 y$ ?

A) $k = - \frac { 1 } { 9 }$
B) $k = \frac { 1 } { 9 }$
C) $k = - \frac { 9 } { 7 }$
D) $k = \frac { 1 } { 7 }$
E) $k = \frac { 9 } { 7 }$