Let,
[tex]\begin{gathered} R=(x_1,y_1)=(-4,1) \\ S=(x_2,y_2)=(-1,9) \end{gathered}[/tex]
The expression to calculate the distance between two points is,
[tex]\begin{gathered} D=\sqrt[]{(x_2-x_1)^2+(y_2-y_1)^2} \\ D=\sqrt[]{(-1-(-4))^2+(9-1)^2} \\ D=\sqrt[]{(3)^2+(8)^2} \\ D=\sqrt[]{9+64} \\ D=\sqrt[]{73} \\ D=8.5\text{ units} \end{gathered}[/tex]
Thus, the length of the line RS is 8.5 units, and the correct option is option C.
