Evaluate the Radical Function at the Given Value of the Variable

Multiple Choice

Evaluate the radical function at the given value of the variable.
- $$f ( x ) = \sqrt [ 4 ] { x - 20 }$$

A) $[ 20 , \infty )$
B) $[ 0 , \infty )$
C) $( - \infty , \infty )$
D) $[ - 20 , \infty )$