Which of the following statements regarding yield spreads is least accurate The:（）

A. option cost in percentage terms can be computed by subtracting the OAS from the zero-volatility spread.

B. nominal yield spread measures the difference between the YTM on a risky bond and the YTM on a Treasury bond of similar maturity.

C. zero-volatility spread is the constant spread that is added to each Treasury spot rate to equate the present value of a bond’s cash flows to the price of an otherwise identical option-free bond.