Find the Least Common Denominator (LCD) $\frac { 3 x } { x - 20 } , \frac { 4 x } { 20 - x }$

Multiple Choice

Find the least common denominator (LCD) .
- $\frac { 3 x } { x - 20 } , \frac { 4 x } { 20 - x }$

A) $12 x$
B) $( x - 20 ) ( 20 - x )$
C) $- 1$
D) $x - 20$ or $20 - x$