Which of the following statements is correct? O a. Callable bonds give bond holders rights to retire the bonds before the maturity date. O b. Assuming a flat yield curve - a coupon bond is sold at par when its current yield equals to its coupon rate. O c. Zero-coupon bonds can be sold at a premium as long as the interest rate is non-negative. O d. Retractable bonds give bond issuers rights to extend the maturity of the bond. O e. Convertible bonds give and issuers rights to convert bonds to common shares