`f:R->R, f(x)` is a differentiable bijective function, then which of the following is true? a) `(f(x)-x)f"(x)<0` b) `(f(x)-x)f"(x)>0` c) `(f(x)-x)f"(x)>0` then `f(x)=f^-1(x)` has no solution d) if `(f(x)-x)f"(x)>0` then `f(x)=f^-1(x)`