Read the passage.

The Dirty Thirties and the Filthy Fifties
The southern Great Plains, an area of the U.S. known for its wheat production, was struck by a severe drought in the early 1930s. Farmers were unable to coax wheat from the parched earth. Huge swaths of plowed land, unanchored by native grasses, were left exposed to the region's strong winds, resulting in years of dust storms that earned the decade the title the Dirty Thirties. Facing poverty and dust inhalation, four hundred thousand people fled the region. Years later, a resurgence of dust storms occurred during another drought. However, the extent of the damage during the Filthy Fifties (as the decade came to be called) offered a contrast to the Dirty Thirties. Not only was the drought shorter, but the land was less vulnerable. In the intervening years, the federal government had restored prairie grasses, while farmers had also applied techniques to prevent soil erosion.
What is the main, or central, idea of the passage?

The main idea of the passage is the comparison between the dust storms and droughts of the Dirty Thirties and the Filthy Fifties, and how various factors such as the vulnerability of the land and government intervention played a role in mitigating the damage.

The central idea of the passage is the comparison between the severe droughts and dust storms in the 1930s, known as the Dirty Thirties, and a resurgence of dust storms in the 1950s, known as the Filthy Fifties, highlighting the contrast in the extent of damage and the measures taken to prevent soil erosion.