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The @ symbol was used to show the cost or weight of something     it became the standard symbol for e-mail．

A．though    B．since    C．when   D．before

The symbol @ was used to show the cost or weight of something __________ it became the standard symbol for e-mails.

A．though      B．since      C．when    D．before

The @ symbol was used to show the cost or weight of something　　 it became the standard symbol for e-mail.

A.though　 B.since　 C.when　 D.before