The correct statement is that "There is a thin layer of topsoil in tropical rainforests."
This is because the tropical rainforest is characterized by constant rainfall. This leads to eating away of the topsoil in the tropical rainforest, thereby making it thin in texture and content.
Option A is not correct because temperate areas have thickly layered topsoil.
Options B and C are wrong because desserts have sandy soil, which is not humus in anyways.
Hence, in this case, the correct answer is option D. "There is a thin layer of topsoil in tropical rain forests."
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