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

Multiple Choice

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

A) $6 \pi$
B) $\frac { 4 \pi } { 3 }$
C) $\frac { \pi } { 4 }$
D) 4