A function $f$ on the set of real numbers is defined for every real number $x$ where $n$ is an integer as
$$f ( x ) = x - n , \quad x \in [ n , n + 1 )$$
Accordingly,
$$f ( 1 ) + f \left( \frac { 7 } { 3 } \right) + f \left( \frac { 13 } { 6 } \right)$$
what is this sum?\
A) $\frac { 1 } { 2 }$\
B) $\frac { 2 } { 3 }$\
C) $\frac { 7 } { 6 }$\
D) 1\
E) 2