The lines $x-2 y+1=0,2 x-3 y-1=0$ and $3 x-y+k=0$ are concurrent. The angle between the lines $3 x-y+k=0$ and $m x-3 y+6=0$ is $45^{\circ}$. If $m$ is an integer, then $m-k=$

• A

  $-6$
• B

  $18$
• Correct answer
  C

  $6$
• D

  $-18$

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