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Let me provide an intuitive answer that I just thought of (correct me if I am wrong). So starting with two Stochastic Differential Equations (SDE) $\frac{dS_t}{S_t}=μdt+σdW_t$ $\frac{dD_t}{D_t}=-rdt$ (I am assuming our risk-free rate to be constant as is done in most introductory financial math courses) Notice: $D_t = e^{-rt}$ is the solution to the ODE ...