Perform the Indicated Operation $( 3 \mathrm { i } ) ( 7 \mathrm { i } )$

Multiple Choice

Perform the indicated operation. Write the result in the form a + bi.
- $( 3 \mathrm { i } ) ( 7 \mathrm { i } )$

A) $21 \mathrm { i }$
B) 21
C) $- 21 \mathrm { i }$
D) $- 21$