Below are the graph of the line $y = x$ and the graph of the function $y = f ( x )$.
Starting from point $\mathbf { Q } ( \mathbf { a } , \mathbf { 0 } )$ and following the arrows, point $\mathbf { P } ( \mathbf { a } , \mathbf { b } )$ is reached. Accordingly, $\mathbf { b }$ is equal to which of the following?\\
A) $a + f ( a )$\\
B) $a \cdot f ( a )$\\
C) $f ( a ) - a$\\
D) $f ( f ( a ) )$\\
E) $f ( a + f ( a ) )$