**Situation**: My opponent has a Zombie Army with 3 +1/+1 counters, and also a another Zombie Army (say for example, [Amoeboid Changeling](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bAmoeboid%20Changeling%5d)). They cast a card with Amass (for example: [Relentless Advance](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bRelentless%20Advance%5d)). I have a [Jaya's Greeting](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bJaya%27s%5d%2b%5bGreeting%5d) in hand.

**Question**: Can I choose to respond to the Relentless Advance by destroying whichever creature will get the tokens? Or does my opponent choose after it resolves?