For the function f(x)=3x, which of the following best describes f′(0)?
a) f′(0)=0 because the tangent line is horizontal at the origin.
b) f′(0)=∞ since f′(x)=3x2/31 and substituting x=0 gives division by zero.
c) f′(0) does not exist because the slope of the secant lines approaches ±∞ as x→0.
d) f′(0) does not exist because f is discontinuous at x=0.