(Centaur Strand isn't a spell.)
When Centaur Strand enters the battlefield, return a land you control to its owner's hand.
All creatures have Swampwalk.
: Add to your mana pool.
: Add or to your mana pool. Each opponent gains 1 life.
: Add 
to your mana pool.: Add 
or 
to your mana pool. Each opponent gains 1 life.
