The correct answer is C. snow line.
The snow line is the lower limit of perpetual snow in mountains. It is the line above which snow remains on the ground year-round. The snow line varies depending on latitude, altitude, and exposure. In the tropics, the snow line is typically at an altitude of 4,500-5,000 meters. In the mid-latitudes, the snow line is typically at an altitude of 2,500-3,000 meters. In the high latitudes, the snow line can be found at sea level.
The snow line is important because it marks the boundary between the alpine and subalpine zones. The alpine zone is the zone above the snow line, where the climate is too cold for trees to grow. The subalpine zone is the zone below the snow line, where trees can grow.
The snow line is also important because it is a sensitive indicator of climate change. As the climate warms, the snow line is expected to move higher in elevation. This will have a significant impact on the alpine environment, as well as on the people who live in the mountains.
The other options are incorrect because they do not refer to the lower limit of perpetual snow in mountains.
- Option A, tree line, refers to the upper limit of tree growth.
- Option B, timber line, is a synonym for tree line.
- Option D, boundary line, is a general term that can refer to any boundary.