The Rule of $72$O a$.$ is used to estimate the amount of time it would take for any given amount of money to double. b$.$ is used to estimate the PV of a FV that has doubled over a fixed time period, given a compounding interest rate.O c$.$ is used to value financial assets that are guaranteed to double within $10$ years at an annual discount rate of $5\%.$ d$.$ is used to determine the risk$-$adjusted rate of return when an investment doubles in value.