Which of the following securities is exempt from registration under the Securities Act of 1933?
a. Shares of nonvoting common stock provided their par value is less than $1.
b. Bonds issued by a municipality.
c. Limited partnership interests sold for the purpose of acquiring funds to invest in bonds issued by the United States.
d. Corporate debentures that were previously subject to an effective registration statement provided they are convertible into shares of common stock.