For a parametric curve given by x=f(t) and y=g(t), which of the following conditions at t=t0 guarantees that the tangent to the curve is vertical?
a) f′(t0)=0 and g′(t0)=0
b) f′(t0)g′(t0)→∞
c) f′(t0)=0 and g′(t0)=0
d) f′(t0)=0 and g′(t0)=0