An analyst is comparing two bonds and has collected the following data: Bond A: Nominal spread of 125 basis points, zero-volatility spread of 125 basis points, and option-adjusted spread of 65 basis points. Bond B: Nominal spread of 95 basis points, zero-volatility spread of 95 basis points, and option-adjusted spread of 95 basis points. Each bond is similar in all respects except that bond A is a mortgage pass-through security and bond B is a non-callable corporate bond. Based only on this information, which of the following statements is most accurate（）
A. The yield curve is perfectly flat.
B. Bond A is preferred over Bond B because its nominal spread is 30 basis points higher.
C. The option-adjusted spread measures the relative curvature of the yield curve and its effect on option cost.
