Let Q be the image of a point $\mathrm{P}(1,2)$ with respect to the line $x+y+1=0$ and R be the image of Q with respect to the line $x-y-1=0$. If M and N are the midpoints of PQ and QR respectively, then $\mathrm{MN}=$

• Correct answer
  A

  $\sqrt{10}$
• B

  4
• C

  $\sqrt{22}$
• D

  5

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