"As families realized that the drought...would not end, some sold what they could not take and began to drive west on Route 66. Many hoped to become hired hands on California farms, learning how to grow fruits and vegetables while living on the farms where they worked. However, California farms typically hired seasonal workers only when they were needed, and used farm workers to perform specific tasks rather than learn how to become farmers in their own right." --John Holland, Modesto Bee, September 30, 2008. Which of the following would have directly caused the events described in the passage above?