The function f(x)=3x is continuous for all real numbers. Which statement best describes the behavior of f′ at x=0?
a) f′(0)=0 because the graph is flat at the origin.
b) f′(0)=∞ since f′(x)=31x−2/3 and substituting x=0 gives division by 0.
c) f′ does not exist at x=0 because the tangent line is vertical.
d) f′ does not exist at x=0 because f is not continuous there.