Which of the Following Equations Would NOT Be Appropriate to Use

Which of the following equations would NOT be appropriate to use in a firm with risky debt?

A) $\beta _ { U } = \frac { E } { E + D } \beta _ { E } + \frac { D } { E + D } \beta _ { D }$
B) $\beta U = \beta _ { E ^ { + } } \frac { D } { E } \left( \beta U - \beta _ { D } \right)$
C) $\beta _ { E } = \beta U + \frac { D } { E } \left( \beta _ { U } - \beta _ { D } \right)$
D) $\beta _ { E } = \beta U + \frac { D } { E } \beta U$