A bag contains $3$ red and $6$ green marbles. If you pick out a marble at random, what isthe chance that it will be red? 
选项为:A:$\dfrac{1}{3}$  B:$\dfrac{1}{2}$  C:$\dfrac{1}{9}$  D:$\dfrac{2}{3}$  请回答其中一个选项.