Electronic mail and filing permit a user to compose and transmit a message on an office (71) system. In the early 1970s, the ARPANET community developed a number of such systems which have been heavily used. Through standard message format (72) , several hundred different computers and electronic mail interfaces are specification of how (73) and return addresses should appear on envelopes and which sizes are allowable for envelopes. In the electronic message world, they describe what (74) of characters are required at the (75) of a message to identify the sending and receiving mailboxes.
A．end
B．last
C．initiation
D．beginning