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As Port Town enters the battlefield, you may reveal a Plains or Island card from your hand. If you don't, Port Town enters the battlefield tapped.
{T}: Add {W} or {U}.
If a Plains or Island is entering the battlefield from your hand at the same time as Port Town, you may reveal the other land to have Port Town enter untapped.
If an effect instructs you to put Port Town onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.
Lands don't have a subtype just because they can produce mana of the corresponding color. Port Town itself is neither a Plains nor an Island, even though it produces white and blue mana, so you can't reveal one to satisfy the ability of another.
You may reveal any land card with either or both of the appropriate subtypes. It doesn't have to be a basic land. For example, you could reveal Canopy Vista from the Battle for Zendikar set to satisfy the ability of Port Town.
