There are 15 balls in a jar. You take out 3 blue balls without putting them back inside, and now the probability of pulling out a blue ball is 1/3. How many blue balls were there in the beginning?
Select the correct option
A) 12 .
B) 9 .
C) 8 .
D) 7 .
E) 6 .

Rationale: "7 = 3 blue balls + 12 / 3 answer : d"
Answer: D