Write the Expression in the Form Bi, Where B Is $\sqrt { - 81 }$

Multiple Choice

Write the expression in the form bi, where b is a real number.
- $\sqrt { - 81 }$

A) $\pm 9$
B) $- 9 \mathrm { i }$
C) $- \mathrm { i } \sqrt { 9 }$
D) $9 \mathrm { i }$