Use the Limit Process to Find the Slope of the Graph

Multiple Choice

Use the limit process to find the slope of the graph of $\sqrt { x + 21 }$ at $( 4,5 )$ .

A) $\frac { 1 } { 5 }$
B) 5
C) $\infty$
D) $\frac { 1 } { 10 }$
E) the slope is undefined at this point