Which of the following series of events is out of order?

a
Overgrazing of cattle led to lower prices, which led to a decline in the cattle industry in Texas.
b
When demand increased after the Civil War, a boom in the sheep industry occurred, which led to conflicts with ranchers and farmers over grazing land.
c
Cattle drives of the late 1800s led to the establishment of large ranches, which led to the development of the railroad.
d
Large profits in the cattle industry led to rapid expansion, which led to a surplus of cattle.