For a function f to be differentiable at x=a, which one of the following statements must be true?
a) f is continuous at x=a
b) Only the right-hand limit of hf(a+h)−f(a) as h→0+ exists
c) Both one-sided limits of hf(a+h)−f(a) as h→0 exist and are equal
d) The sum of the two one-sided limits of hf(a+h)−f(a) equals 0