Which of the following statements about debt securities is least accurate（）
A. Commercial paper is a short-term (less than nine months) vehicle for corporate borrowing.
B. A MTN is a shelf-registered debt instrument that is continually offered to investors by an agent of the issuer and varies in maturity from nine months to over 30 years.
C. A medium-term note (MTN) differs from a corporate bond in that a MTN is sold to investors on a "firm commitment" basis wherein the investment banker guarantees a price to the issuer.