Which of the Following Is Not a True Statement

Which of the following is not a true statement?

A) Mortgage bonds are debt instruments that are secured by real assets.
B) Callable bonds give the issuer the option to "call" or repurchase outstanding bonds at predetermined call prices at specified times.
C) Retractable bonds allow the bondholder to sell the bonds back to the issuer at predetermined prices at specified times earlier than the maturity date.
D) Extendible bonds allow the issuer to extend the maturity date of the bond.