scientific theory is an explanation of some aspect of the natural world that can, in accordance with the scientific method, be repeatedly tested, using a predefined protocol of observations and experiments.
When Napoleon III became emperor in 1852, Paris was the biggest city in Continental Europe. It was the center for commerce, fashion, art and finance. It has beautiful buildings but these were surrounded by slums. Napoleon III began to solve the main problems of the city: the water shortage, overcrowded slums, sewer system and the narrow roads, making Paris the most beautiful city in Europe.
The one that was a factor in the U.S entry in world war I was :
B. the sinking of the Lusitania