What is the most important difference between a corporate bond and an equivalent US Treasury Bond?

A. Corporate cash flow is relatively smooth, whereas US Gov't revenue is more variable.

B. Corporate bonds are traded on the floor of the NY Stock Exchange and Treasury Bonds trade in the Over the Counter market.

C. In the case of corporate bonds, firms have sometimes defaulted, whereas the US Gov't has not.

D. The beta of corporate bonds is usually less than the beta of a US Treasury Bond.