Excerpt from Sacramento Valley Agriculture
Sacramento History Online

9 World War I precipitated the rapid development of trucks and tractors. Farmers increased their production to help feed the armies. After the war, crop prices collapsed and land prices rose. The National Prohibition Amendment in 1919 had a significant impact in the Sacramento Valley. Farmers began to pull out vineyards and convert their land to other crops. The two largest vineyards in the nation (Stanford’s Vina Ranch and the Natomas vineyards south of the American River) disappeared.
In paragraph 9, readers can conclude that crop prices fell and land prices rose because
A) the vineyards were not good farm land.
B) no one wanted to live in the Sacramento Valley anymore.
C) there was an enormous crop failure due to draught in 1919.
D) farmers were no longer growing extra crops to feed soldiers.