Which part of the steam engine does the work?

Imagine living off nothing but coal and water and still having enough energy to run at over 100 mph! That's exactly what a steam locomotive can do. Although these giant mechanical dinosaurs are now extinct from most of the world's railroads, steam technology lives on in people's hearts and locomotives like this still run as tourist attractions on many heritage railways.

Steam locomotives were powered by steam engines, and deserve to be remembered because they swept the world through the Industrial Revolution of the 18th and 19th centuries. Steam engines rank with cars, airplanes, telephones, radio, and television among the greatest inventions of all time. They are marvels of machinery and excellent examples of engineering, but under all that smoke and steam, how exactly do they work?

the answer is b ) square root of 5