The correct answer is - D. the invention of the automobile.
The invention of the automobile was revolutionary for everyone. As the automobiles developed more and more, and were becoming faster and more efficient, they became a transportation mean that was suitable for everyone. Also, the prices of the automobiles was not too high, so lot of people were able to afford them. With it, the leisure travel developed as well. The reason for that was that it was easy to travel over long distance in relatively short time, and it was also relatively cheap. The people started using it more and more, as they wanted to see new things, to go on vacations, to experience new things, and to enjoy.
