71.

A point-to-point communication mode is seen in

• satellite cable communication

• television transmission

• fax transmission

• FM radio transmission

If the heights of transmitting and the receiving antennas are each equal to h, the maximum line-of-sight distance between them is (R is the radius of the earth)

• $\sqrt{2 \mathrm{Rh}}$

• $\sqrt{4 \mathrm{Rh}}$

• $\sqrt{6 \mathrm{Rh}}$

• $\sqrt{8 \mathrm{RH}}$