If you buy the first WoW expansion,would you be playing in the Cataclysm world?

[Jacey Barrett]

--If I buy the first one,would I still be able to play in the,new,cataclysm world,just no content past TBC?Such as not being able to get past level 60?

--Oh and,If I have a trial,could I still play with it once I buy the first WoW?(If the question above is right.)If not,then do I have to wait till I get cataclysm to play on my trial account,or not at all?

Thanks.