Answer: 1. It introduces dangers that are not visible.
Explanation: In this passage from Part 1 of "To Build a Fire" by Jack London, this setting detail advances the plot because <u><em>it introduces dangers that are not visible</em></u>. The protagonist is traveling the Yukon to find gold. He has to meet his friends at six. In spite of not knowing anything about the place and the effects of the cold weather and thick snow, he sees no danger ahead. The dog does not want to continue but is forced to do so. The icy cover of the river breaks when it passes. This detail shows that there are dangers that are not visible for the man.
