Investing.com - Top cryptocurrency exchange Coinbase says it may add trading in five more coins.The coins under consideration are Cardano, Basic Attention Token, Stellar Lumen, Zcash and 0X.Coinbase says that at this point it is only exploring the addition of the five coins, but made the announcement to be transparent about its business.When Coinbase added Bitcoin Cash to its offering last December, its price soared in advance, triggering questions about insider trading.In this case, Coinbase says the currencies being considered will require "additional exploratory work" and there is no guarantee they will "be listed for trading."Coinbase currently offers trading in Bitcoin, Bitcoin Cash, Litecoin and Ether in the U.S. and 32 other countries.The trading app Robinhood recently said it was adding four new currencies to its offering, giving it a total of six.