Electromagnetic waves with frequencies greater than the critical frequency of ionosphere cannot be used for communication using sky wave propagation, because

Assertion: Optical fibres are free from electromagnetic disturbances.

Reason: Optical fibres are electrically insulated.

Assertion: Optical communication system is more economical than other systems of communications.

Reason: The information carrying capacity of a communication system is directly proportional to its band width.

Assertion: The electrical conductivity of the earth's atmosphere increase with altitude

Reason: The high energy particles (i.e. $\gamma$ -rays and cosmic rays) coming from outer space while entering the earth's atmosphere cause ionization of the atoms of the gasses present in the atmosphere.

Assertion: The receiver reconstructs the original message or data after its propagation through communication channel.

Reason: The reconstruction is necessary only in some cases.

Assertion: Fax is a modulating and demodulating device.

Reason: It is necessary for exact reproduction of a document

Assertion The electromagnetic waves of longer wavelengths can travel longer distances on the earth's surface than those of shorter wavelengths.

Reason Velocity of wave propagation is same for all wavelengths.

Assertion: Modulator or encoder is an essential component of a transmitter.

Reason: The message signal has to be changed into a form suitable for transmission.

Assertion: Ground wave transmission becomes weaker with increase in frequency.

Reason: The ground waves can bend round the corners of the object on the earth.