A real valued function $f\left(x\right)$ satisfies the functional equation $f\left(x-y\right)=f\left(x\right)f\left(y\right)-f\left(a-x\right)f\left(a+y\right)$  where a is a given constant and $f\left(0\right)=1,f\left(2a-x\right)$  is equal to

• A

  $-f\left(x\right)$

• B

  $f\left(x\right)$

• C

  $f\left(a\right)+f\left(a-x\right)$

• D

  $f\left(-x\right)$