Which of the following statements correctly defines a horizontal asymptote of the graph of a function f?
a) The line x=L is a horizontal asymptote of f if x→Llimf(x)=±∞.
b) The line y=L is a horizontal asymptote of f if x→±∞limf(x)=L.
c) The line y=L is a horizontal asymptote of f if x→Llimf(x)=±∞.
d) The line y=L is a horizontal asymptote of f if x→±∞limf(x)=±∞.