With Respect to Currency Measurement,what Is the Rule When Discounting

With respect to currency measurement,what is the rule when discounting cash flows?

A) The revenues and expenses must be measured as they occur rather than on an incremental basis.
B) There is no need to incorporate the risk premium of that the firm's stockholders would demand in the discount rate.
C) Revenues and expenses should be measured on a pre-tax cash flow basis.
D) All cash flows should be measured in the same currency.