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

• A

  $f\left(x\right)$

• B

  $-f\left(x\right)$

• C

  $f(-x)$

• D

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