填空题
A—abbreviated dialing code
B—off-peak hours
C—charging period
D—access code
E—identity number
F—video conference
G—operational status
H—information subscription service
I—Network User Address
J—audio signal
K—local user terminal
L—file management
M—response signal
N—operating instructions
O—change of the battery unit
P—function indicator
Q—entry rejected
R—external control