Which of the following function is surjective but not injective. (a) `f:R->R ,f(x)=x^4+2x^3-x^2+1` (b) `f:R->R ,f(x)=x^2+x+1` (c) `f:R->R^+ ,f(x)=sqrt(x^2+1)` (d)`f:R->R ,f(x)=x^3+2x^2-x+1`