Oracle Text

You may put a creature card from your hand onto the battlefield. If you do, sacrifice it unless you pay its mana cost reduced by up to 2.

Card Rulings

3/16/2018 If the creature has in its mana cost, X is considered to be 0.
3/16/2018 Only the generic mana in that creature’s mana cost is reduced. For example, if that creature’s mana cost is , you’ll have to pay to keep it.
3/16/2018 The creature enters the battlefield, so it will trigger enters-the-battlefield abilities even if you choose not to pay.
3/16/2018 If you choose not to pay, the creature is sacrificed immediately. No player will get priority in between the creature entering the battlefield and being sacrificed. Sacrificing the creature this way will trigger any abilities that trigger when it leaves the battlefield, and those abilities will be put onto the stack at the same time as those that triggered when it entered the battlefield.