In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet. Communicating via e-mail is by far the most common Intemet activity. You can （）with anyone in the world who has an Intemet address or e-mail （）with a system connection to the Internet. All you need is （）the Internet and an e-mail program. Two of the most widely used e-mail programs are Microsoft’s （） and Netscape’s Navigator. A typical e-mail message has three basic （）: header, message, and signature.

Two of the most widely used e-mail programs are Microsoft’s（）and Netscape’s Navigator.

A．Yahoo

B．Explorer

C．Foxmail

D．Outlook Express

不属于天然水净化混凝剂的是 [ ]

A．硫酸铝

B．硫酸亚铁

C．硫酸镁

D．聚合铝