4. (4 points) Find the least integer n such that f(x) is O(xn) for each of these functions. (a) f(x)=2x2+x3logx (b) f(x)=3x5+(logx)4 (c) f(x)=(x4+x2+1)/(x4+1) (d) f(x)=(x3+5logx)/(x4+1)