Over time China has changed the modification of physical environments so they can develop productive agricultural systems . they have also modified it by taking lots of land and taking it down to build homes and cities. By doing this they have more cites and homes for people. China has also modified it's country by building the great wall of China and if people visit that's the most popular place to go when you are in china. China build the great wall of china because first the northern tribes tried to invade China.Second most of the time they like to rob something using troops. They appear every where and run very fast and it is hard to catch using their army so they build the great wall of China to keep invaders out.