「何か燃えてる臭いがしないか？」

業火の罠

このターンに あなたが２体以上のクリーチャーからダメージを与えられていたなら、業火の罠のマナ・コストを支払うのではなく、{R}を支払ってもよい。
クリーチャー１体を対象とする。業火の罠はそれに４点のダメージを与える。

Inferno Trap

If you've been dealt damage by two or more creatures this turn, you may pay {R} rather than pay this spell's mana cost.
Inferno Trap deals 4 damage to target creature.

Duel Decks: Speed vs. Cunning

If you've been dealt damage by two or more creatures this turn, you may pay {R} rather than pay Inferno Trap's mana cost.
Inferno Trap deals 4 damage to target creature.

Archenemy

Zendikar

Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.

Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.

Inferno Trap's alternative cost condition checks for any damage, not just combat damage.

The alternative cost condition doesn't check who controlled the creatures that dealt damage to you. Damage dealt to you by your own creatures counts.

The alternative cost condition is satisfied only if two different creatures dealt damage to you, not if one creature dealt damage to you twice. The damage may have been dealt to you simultaneously (as with combat damage), or at separate times.

You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.