Give the Amplitude or Period as Requested $y = \cos \frac { 1 } { 3 } x$

Give the amplitude or period as requested.
-Period of $y = \cos \frac { 1 } { 3 } x$

A) $\frac { 2 \pi } { 3 }$
B) $\frac { \pi } { 3 }$
C) $6 \pi$