which of the following is a rational function
a. f(x)=0
b. f(x)= (3x+1)/(x^2-4x+5)
c. f(x)=e^x
d. f(x)=sinx/cosx
e. f(x)=-4x^-3+5x^-1+7-18x^2
f. f(x)=x^(1/2)-x+8
g. f(x)=x^(1/2)/x^3-x