Which statement about a function f at x=a is always true?
a) If f is continuous at x=a, then f is differentiable at x=a.
b) If f has a removable discontinuity at x=a, then f is differentiable at x=a.
c) If f is differentiable at x=a, then f is continuous at x=a.
d) If the tangent line to f is vertical at x=a, then f is differentiable at x=a.