Question

a majority of the cheesemaking plants in the united states are located in the midwest; wisconsin alone boasts more than one hundred creameries. some cheese experts claim the best cheese made in america comes from this region. why are midwestern cheeses so good? one reason is that grass grows well there, so cows can graze year-round. the milk of grass-fed cows has a distinctive, earthy flavor. in addition, limestone rock is prevalent in the midwest. when water flows through limestone, the limestone filters iron out of the water, resulting in a sweeter taste. cows drink this water and eat grass nourished by the water, so they produce sweeter milk.
what is the main, or central, idea of the passage?
when water is filtered through limestone, it takes on a sweet taste.
because of conditions in the midwest, cheeses produced there have a distinctive, sweet taste.
because grass grows well in the midwest, cows milk from the region has a distinctive taste.

Explanation:

The passage first notes the Midwest's prominence in US cheesemaking, then explains two regional conditions (good grass growth and limestone-filtered water) that create the distinct, sweet taste of its cheeses. The other options only cover single supporting details, not the overarching central idea.

Answer:

Because of conditions in the Midwest, cheeses produced there have a distinctive, sweet taste.