Which of the following conditions is sufficient to conclude that the line x=c is a vertical asymptote of the graph of a function f?
a) x→c−limf(x)=x→c+limf(x), both limits finite
b) At least one of x→c−limf(x) or x→c+limf(x) equals ±∞
c) x→climf(x)=0
d) x→climf′(x)=±∞