15. [5] It's not so difcult to incorporate time-varying volatility into the BSM model as long as the time variation is not random. Assume a BSM economy, but this time, assume that the volatility of the stock varies over time deterministieally: or is not a constant but a (bounded) function of time. Derive the price of a European call. For simplicity, assume that today is time 0; that is, the remaining time to maturity is simply T and not T t. Hints: o The price of a European call when the volatility is constant is SO