13. What does the term "diffusion" refer to in the context of stochastic processes in finance? (A) The tendency of assets to revert to their mean value. (B) The expected difference between the value of the asset and the value of the derivative asset. (C) The process of option pricing using the Black-Scholes model. (D) The rate of change of the underlying asset price. (E) The random fluctuations in asset prices over time.