If $f : [ 0 , \infty ) \rightarrow \mathbb { R }$ is a continuous function such that
$$f ( x ) + \ln 2 \int _ { 0 } ^ { x } f ( t ) d t = 1 , x \geq 0$$
then for all $x \geq 0$,\\
(A) $f ( x ) = e ^ { x } \ln 2$.\\
(B) $f ( x ) = e ^ { - x } \ln 2$.\\
(C) $f ( x ) = 2 ^ { x }$.\\
(D) $f ( x ) = \left( \frac { 1 } { 2 } \right) ^ { x }$.