A bag initially contains red marbles and blue marbles only, with more blue than red. Red marbles are added to the bag until only 1 / 3 of the marbles in the bag are blue. Then yellow marbles are added to the bag until only 1 / 5 of the marbles in the bag are blue. Finally, the number of blue marbles in the bag is doubled. What fraction of the marbles now in the bag are blue?
Answer Choices
A. \dfrac{1}{5}
B. \dfrac{1}{4}
C. \dfrac{1}{3}
D. \dfrac{2}{5}
E. \dfrac{1}{2}