In the following essay,each blank has four choices．Choose the best answer and writedown on the answer sheet．
Communicating via E—mail is by far the most common Internet activity．You can (71) with anyone in the world who has an Internet address or E—mail (72) with a systemconnection to the Intemet．All you need is (73) the Internet and an E—mail program．Two ofthe most widely used E．mail programs are Microsoft’s (74) and Netscape’S Navigator．Atypical E．mail message has three basic (75) ：header，message，and signature·
A．packets
B．elements
C．frames
D．cells