Culling the Weak

• WWizards of the Coast2013-04-15
  Official Ruling
  Players can only respond once this spell has been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to prevent you from casting this spell.
• WWizards of the Coast2013-04-15
  Official Ruling
  You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.

Rulings via Scryfall