Which would be the best title for the chart below?

Safety valve for unhappy workers
and farmers in the Northeast

Discovery of gold and silver in
Colorado, Nevada, and the Dakotas

Advertising promised superior farming conditions on the Great Plains

Building of the
transcontinental railroads

Market for grain and cattle
in the Northeast

Passage of the Homestead Act
provided cheap land

A. Reasons for the Success of the Nebraska Land Rush
B. Factors Contributing to the Settlement of the West
C. Conditions Leading to the Spread of Cholera
D. Sparks for the Second Industrial Revolution