The answer is: [E]: "case study" .
_________________________________________________
The correct answer is - fertile soil.
The Great European Plain, also referred to as the Russian Plain, is dominated by lowlands, naturally covered with dense grasses, and is known for its fertile soil. Most of the soil in this large plain is has very rich and deep upper, humus layer. The reason for this is that the grasses that grow their die out each year, and as they die out they decompose very quickly, giving the soil a new layer of decomposed biomass each year.
Because of the properties of the soil, this the region of Europe that is heavily used for farming, mostly crops like the wheat, hops, and corn. The conditions are excellent for them, they do not take a lot of the soil, and also contribute to keeping the soil very fertile constantly with their decomposing roots and steams.
