Bellows Breath

Q. You play Bellow's Breath repeated, targeting Stellacorn Herder and 2 Traveling Merchants at Void Gate. Your Opponent plays Flash in response, returning the 2 Traveling Merchants to base. What happens next (other than your opponent discarding 1, drawing 1 twice)?

You choose either the Herder or the two Merchants to die.

352.11.b
If the group of targets no longer collectively fulfill the targeting restriction as the spell or ability resolves, that spell or ability's controller can choose a subset of the original targets that fulfills the targeting requirement for the spell or ability to affect.