If they don't answer, an opponent chooses one
For example, if a Lakeside Shack and a Breeding Pool you control are entering at the same time (perhaps because of the effect of a spell like Scapeshift), your life total is 14, and your opponent's life total is 20, the Lakeside Shack will enter tapped whether or not you choose to pay life as Breeding Pool enters
If the spell has any targets, you may choose new targets for any of the copies." As the replicate triggered ability resolves, you'll copy Shattering Spree for each time you paid its replicate cost, even if the original spell is no longer on the stack at that time (perhaps because it was countered)
Which is its current price
Of course, Boromir can also sacrifice himself to give creatures that you control indestructible until end of turn a nd make the Ring tempt you