Many factors have contributed to increase in life expectancy. Examples such as medical advancements in modern medicine, less wars being fought, improvement in sanitation, eating a healthy diet, a overall healthier lifestyle (for example, people don’t smoke cigarettes as much as the olden days) and easier access to education.
The correct answer is - biomass from the decomposing grasses.
The Midwest is a region known for its highly fertile soil. The reason for this is that the weathered bedrock combines with huge amounts of biomass. The biomass is almost entirely form the grasses. The grasses grow naturally in this region and they dominate the landscape. They grow and die out relatively quickly, as most of those grasses are one season grasses. They manage to create very large amounts of biomass every year, and also they decompose very quickly. The grasses also are high in nutrients, so once decomposed, they give the soil a lot of nutrients as well, which makes it very fertile and suitable for agriculture.
It would help to attach a picture or explain the question better. How else are we supposed to know the places the question is talking about?
