The following data represents the frequency distribution of 20 observations

$x_i$	3	4	5	8	10	11
$f_i$	$\alpha+2$	$(\alpha-1)^2$	4	$\alpha-1$	2	$\alpha$

Then its mean deviation about the mean is

• A

  3
• B

  2.4
• Correct answer
  C

  2.7
• D

  2.9

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