A Wave Traveling on a String Is Described by the Expression

Multiple Choice

A wave traveling on a string is described by the expression $y ( x , t ) = 5.0 \cos \left( \frac { \pi } { 3 } x + \frac { \pi } { 6 } t - \frac { 3 \pi } { 2 } \right)$ cm where $x$ is measured in meters and $t$ in seconds. The phase difference at the same point on the string over a time difference of 3.0 s is

A) $\pi$ /6 rad.
B) $\pi$ /3 rad.
C) $\pi$ /2 rad.
D) $\pi$ rad.
E) 2 $\pi$ rad.