Find the Probability of the Event If a Single Choice $\frac { 4 } { 21 }$

Find the probability of the event if a single choice is made from a bag.
-A bag contains 8 red marbles, 4 blue marbles, and 9 green marbles. What is the probability of choosing a blue marble when one marble is drawn?

A) $\frac { 4 } { 21 }$
B) $\frac { 1 } { 3 }$
C) $\frac { 8 } { 21 }$
D) $\frac { 3 } { 7 }$