确保系统稳定性为什么重要，理由不包括（）

A.客户最关注

B.工作出发点

C.投入最大

D.业务系统不能断

Electronic mail, or mail, as it is known to its many fans, has been around for over two decades. Typically, email systems support five basic functions, as described below. (41) refers to the process of creating messages and answers. (42) refers to moving messages from the originator to the recipient. (43) has to do with telling the originator what happened to the message. Was it rejected Was it lost (44) incoming message is needed. So people can read their email. (45) is the final step and concerns what the recipient does with the message after receiving it.

（44）是（）

A.Composition

B.Transfer

C.Reporting

D.Displaying