Final answer:
The Second Industrial Revolution was spurred by the ability to produce steel cheaply, leading to breakthroughs in technology and a greater demand for raw materials.
Step-by-step explanation:
The trigger for the Second Industrial Revolution was c. Innovators figured out how to produce steel cheaply. The development of modern steel in 1856, along with electrical generators in 1870, and the introduction of bicycles and automobiles by the 1890s, marked a period of significant technological advancements. Important inventions such as the telephone by Alexander Graham Bell in 1876 and the mass production of machines were made possible by these innovations. The demand for raw materials such as rubber, mineral ores, and cotton surged due to the need to support the components of new technologies. Advances like the Bessemer process enabled mass steel production which was crucial for the expansion of industries and the construction of modern city skylines.