题目内容：
Electronic MailDuring the past few years, scientists all over the world have suddenly found themselves pro-ductively engaged in task they once spent their lives avoiding--writing, any kind of writing, but particularly letter writing. Encouraged by electronic mail's surprisingly high speed, convenience and economy, people who never before touched the stuff are regularly, skillfully, even cheerfully tapping out a great deal of correspondence.
Electronic networks, woven into the fabric of scientific communication these days, are the route to colleagues in distant countries, shared data, bulletin boards and electronic journals. Any-one with a personal computer, a modern and the software to link computers over telephone lines can sign on. An estimated five million scientists have done so with more joining every day, most of them communicating through a bundle of interconnected domestic and foreign routes known col-lectively as the Internet, or net.
E-mail is starting to edge out the fax, the telephone, overnight mail, and of course, land mail. It shrinks time and distance between scientific collaborators, in part because it is conven-iently asynchronous (异步的) (Writer can type while their colleagues across time zones sleep;their message will be waiting). If it is not yet speeding discoveries, it is certainly accelerating communication.
Jeremy Bernstein, the physicist and science writer, once called E-mail the physicist's umbili-cal cord (脐带)。 Later other people, too, have been discovering its connective virtues. Physi-cists are using it; college students are using it; everybody is using it; and as a sign that it has come of age, the New Yorker has celebrated its liberating presence with a cartoon--an apprecia-tive dog seated at a keyboard, saying happily, "On the Internet, nobody knows you're a dog. "
The reasons given below about the popularity of E-mail can be found in the passage except _________. A．direct and reliable
B．time-saving in delivery
C．money-saving
D．available at any time
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