(A) For continuous random variables, the cumulative distribution function (CDF) is the integral of the probability distribution function (PDF) with respect to x.
(B) For discrete random variables, the CDF is the sum of probabilities up to a given point x.
(C) For continuous random variables, the PDF is the derivative of the CDF.
(D) For discrete random variables, the PDF is the difference between successive CDF values.
Therefore, option (C) is correct.