15. [5] It's not so difficult 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 deterministically: 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: The price of a European call when the volatility is constant is -rT So(d1) Ke (d2) where d = log()+(r+o)T and d = log()+(r-o)T Use the risk-neutral valuation approach, not the PDE approach.