Answer :

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flat surfaces on a rectangular prism are called faces..
reason:
it is so because those surfaces are of polyhedron shape means that their spherical surfaces are in front and they have a general outward appearance .. Face is a Latin word( facies) meaning in front...
polyhedron shape is like:

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luana
[tex]In\ geometric\ objects\ faces\ are\ surfaces\ of\ some\ objects\ which\ can\\\\be\ viewed\ and\ meet\ at\ edges.\\\\ Rectangular\ prism\ has\ surfaces\ which\ can\ be\ seen. \ Between\\\\surfaces\ always\ there\ are\ edges. \\\\Secondly\ face\ have\ to\ be\ flat\ surface.\ Every\ solid\ figure\ has\\\\faces.\ They\ vary\ only\ in\ shape.\ Rectangular\ prism\ has\ four\ flat\\\\ rectangular\ surfaces \ and\ two\ square\ flat\ surfaces. Each\ of\ them\\\\fulfill\ conditions\ for\ being\ a\ face. [/tex]