计算机网络信息安全中防火墙通常是指在网络安全边界控制中， 用来阻止从外网想进入给定网络的()对象的安全设备。
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Sometimesabus____getsonthebustocheckthetickets.
At______,photographyisachemicalprocess,duringwhichalight-sensitivematerialisalteredwhenexposedtolight.
Idon’tunderstand____believehim.
The____populatedareawasabreedingplaceforinfectiousdiseases.
Writing：
Anoldsayinggoes“hastmakeswaste”.Writeacompositionofabout400wordstostateyourviewonit.
