If the mean of two samples of sizes 20 and 30 are 25 and 10 respectively, their variances are 9 and 16 respectively, their combined variance is

• A

  $67.9$
• B

  $68.2$
• C

  $68.1$
• Correct answer
  D

  $67.2$

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