Find All Solutions of the Given Equation in the Interval $$\cos 3 x = \cos x$$

Multiple Choice

Find all solutions of the given equation in the interval [0,2π) . ​​ $$\cos 3 x = \cos x$$ ​

A) ​ $x = 0 , \frac { \pi } { 2 } , \pi , \frac { 3 \pi } { 2 }$
B) ​ $x = \frac { \pi } { 4 } , \frac { \pi } { 2 } , \frac { 3 \pi } { 2 }$
C) ​ $x = 0 , \frac { \pi } { 4 } , \frac { \pi } { 2 } , \pi$
D) ​ $x = 0 , \frac { \pi } { 4 } , \pi , \frac { 3 \pi } { 4 }$
E) ​ $x = \frac { \pi } { 8 } , \frac { 2 \pi } { 8 } , \frac { \pi } { 2 } , \frac { 3 \pi } { 8 } , \pi , \frac { 3 \pi } { 2 }$