If $\sum_{i=1}^9\left(x_i-5\right)=9$ and $\sum_{i=1}^9\left(x_i-5\right)^2=45$, then the standard deviation of the nine observations $\mathrm{x}_1, \mathrm{x}_2, \ldots, \mathrm{x}_9$ is

• Correct answer
  A

  $2$
• B

  $4$
• C

  $3$
• D

  $9$

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