The function f is given by
$$\mathrm { f } ( x ) = \frac { \cos x + 3 } { 7 + 5 \cos x - \sin ^ { 2 } x }$$
Find the positive difference between the maximum and the minimum values of $\mathrm { f } ( x )$.
A 0
B $\frac { 1 } { 3 }$
C $\frac { 1 } { 2 }$
D $\frac { 2 } { 3 }$
E 1
F 2