A Ball That Is Bobbing Up and Down on the End

Multiple Choice

A ball that is bobbing up and down on the end of a spring has a maximum displacement of 3 inches.Its motion (in ideal conditions) is modeled by
$y = \frac { 1 } { 4 } \cos 18 t ( t > 0 )$
where y is measured in feet and t is the time in seconds.
What is the period of the oscillations ?

A) $\frac { 9 } { \pi }$
B) $\frac { 18 } { \pi }$
C) $\frac { \pi } { 18 }$
D) $\frac { \pi } { 2 }$
E) $\frac { \pi } { 9 }$