In Which of the Following Situations Would It Not Be

In which of the following situations would it not be appropriate to use the following formula: PV = C₁/(1 + r₁) + C₂/(1 + r₂) ² + ....+ Cn/(1 + rn) ⁿ
When determining the present value (PV) of a cash flow stream?

A) when yield curves are flat
B) when short-term and long-term interest rates vary widely
C) when the inflation rate is high
D) when the discount rate is high
E) when the real interest rate is high