Which of the following statement is correct ?

• A. Satellite transponders contain a device that echos the radiation without change from one point on earth to another
• B. satellite transponders us lower frequency reception of radiation from earth stations and higher frequency for transmission to earth station
• C. Satellite transponders use a higher frequency for reception of radiation from earth stations and lower frequency for transmission to earth stations
• D. Satellite transponder contain devices that transform the message sent from one location on earth to a different code for transmission to another location