单项选择题
A.daily testing
B.either A or C
C.operational use of the equipment
D.testing at least every 48 hours
单项选择题 （） operates at a transfer rate of up to 9600 bits per second and is telephone，telex and facsimile（fax）capable.
单项选择题 （） is not required by bridge-to-bridge communications.
单项选择题 （）is an area within the radiotelephone coverage of at least one VHF coast station in which continuous Digital Selective Calling is available.