Which maintenance functions can a GMDSS Radio Operator perform（）.

A. The Operator can make fine internal adjustments to the transmitter as long as the output power does not change by more than one percent
B. The Operator is responsible for ensuring that INMARSAT antennas are free of built-up soot and clear of obstacles
C. All levels of maintenance must be performed by a licensed GMDSS Radio Maintainer
D. The Operator may install an EPROM in order to ensure that the equipment continues to operate within legal constraint