Text (CK+errata): Whenever a player taps a land for mana, that player adds one mana to his or her mana pool of any type that land produced. [Oracle 2008/04/01]
This is one additional mana. [D'Angelo 2005/12/01]
If the land produces more than one type or mana, the player can choose which type the additional mana is. [D'Angelo 2005/12/01]
