Which of the following formulas is used to calculate the discounting factor for internal rate of return $($IRR$)$ when an investment produces a series of uniform cash flows?

a$.$ Discounting Factor $=($Total Annual Cash Flow $+$ Investment$)/$ Annual Cash Flow

b$.$ Discounting Factor $=$ Investment $/$ Annual Cash Flow

c$.$ Discounting Factor $=$ Annual Cash Flow $/$ Investment

d$.$ Discounting Factor $=($Total Annual Cash Flow $$ Investment$)/$ Annual Cash Flow