Which of the following statements is always true?
◦ If f is continuous at c, then it must be differentiable at c.
◦ If
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f(c + h) = f(c), then f must be differentiable at c.
◦ If f is not differentiable at c, then it must be discontinuous at c.
◦ If f is differentiable at c, then it must be continuous at c.
◦ All of the above