Communication circuit <em>(D)</em> is becoming more common in residential electrical design and construction.
LAN Ethernet cables, outlets, and even hubs and bridges, are being built into the walls of new homes, along with the usual electrical outlet wiring, to give the owner the networking infrastructure and internet access that everybody needs now ... without stringing a mess of cables on the floor and through doors all over the house.
Sound travels fastest in solids,than liquids, and faster in liquids, than in gases.
