Downhill Charge 

Instant
You may sacrifice a Mountain rather than pay this spell's mana cost.
Target creature gets +X/+0 until end of turn, where X is the number of Mountains you control.
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Cards that do something similar expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Mine Collapse 96%
$0.19#12890
"If it's your turn, you may sacrifice a Mountain rather than pay this spell's mana cost."
Mogg Alarm 85%
$0.23#18432
"You may sacrifice two Mountains rather than pay this spell's mana cost."
The First Eruption 82%
$0.15#22762
"Sacrifice a Mountain. If you do, this Saga deals 3 damage to each creature."