Let a be a real number. A function f is defined on the set of real numbers as
$$f ( x ) = \left\{ \begin{array} { c c c }
a - x & , & x < 1 \\
5 x - 4 & , & 1 \leq x \leq 5 \\
( x - a ) ^ { 2 } + 12 & , & x > 5
\end{array} \right.$$
If there is only one point where the function f is not continuous, what is the value of
$$f ( 7 ) - f ( 0 )$$?\\
A) 3\\
B) 4\\
C) 5\\
D) 6\\
E) 7