There are two boxes each containing 10 balls. In each box, few of them are black balls and rest are white. A ball is drawn at random from one of the boxes and found that it is black. If the probability that the black ball drawn is from the second box is $\frac{1}{5}$, then number of black balls in the first box is

• A

  5 or 10
• B

  2 or 7
• Correct answer
  C

  4 or 8
• D

  3 or 6 or 9

Hence, the correct answer is option: 'C'.