In a shelf there are three mathematics and two physics books. A student takes a book randomly. If he randomly takes, successively for three times by replacing the book already taken every time, then the mean of the number of mathematics books which is treated as random variable is

• A

  $\frac{3}{2}$
• B

  $\frac{129}{125}$
• Correct answer
  C

  $\frac{9}{5}$
• D

  $\frac{174}{125}$

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