The correct answers are The explorers must have found a hidden ice cave, They were incredibly lucky to stumble upon an abandoned trapper's cabin, and The guides had wisely brought a supply of emergency thermal blankets.

The word "otherwise" acts as an implied conditional meaning "if this had not happened." Because the result clause (would have perished) is a past unreal conditional (third conditional), the first clause must describe a completed past event that actually happened and saved them.

"The team has packed..." is incorrect because the present perfect tense (has packed) clashes with the past narrative context.

"They failed to secure..." is grammatically fine but logically absurd—it implies that failing to find shelter is what saved their lives!