What is the main currency used for trade in WOW?

Gold, Silver and Copper
Rubies
Dragon skins
Mana points

Info: Any item can be bought with gold, silver or copper. The main currency for buying and selling goods is gold. To get one golden coin, a player has to collect 100 silver coins. 1 silver coin is equivalent to 100 copper coins. The game also allows exchanging items between players.