Let $f: R \rightarrow R, f(x-f(y))=f(f(y))+x f(y)+f(x)-1 \forall x$,
$y \in R$, if $f(0)=1$ and $f^{\prime}(0)=0$, then
(A) $\mathrm{f}(\mathrm{x})=1-\frac{\mathrm{x}^{2}}{2}$
(B) $f(x)=x^{2}+1$
(C) $f(x)=\left(\frac{2 x+1}{x+1}\right)$
(D) none of these