Question

1. why are topsoil layers the thickest at the bottom of a mountain?rainwater washes the topsoil down the mountain.it is warmer at the bottom of the mountain, so more topsoil forms.the bedrock is different at the bottom, so more topsoil forms.there is less subsoil, so more topsoil forms.

Explanation:

Topsoil is eroded from upper mountain slopes by rainwater runoff, which carries this loose, nutrient-rich soil material downhill and deposits it at the mountain's base, leading to a thicker topsoil layer there. The other options are incorrect: temperature differences do not directly drive thicker topsoil formation, bedrock type does not inherently create more topsoil at the base, and subsoil quantity is not the causal factor.

Answer:

Rainwater washes the topsoil down the mountain.