The transmitting antenna placed at the top of a tower has a height of $45 \mathrm{m}$ from the ground. The distance between receiving and transmitting antennas is $40 \mathrm{km}$ and the radius of earth is $6400 \mathrm{km}$. The minimum height (in $m$ at which the receiving antenna is to be placed for satisfactory communication in $LOS$ mode, is

• A

  $5$
• B

  $15$
• Correct answer
  C

  $20$
• D

  $25$

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