Shanghai gained importance due to its prime location at:

Shanghai gained importance due to its prime location at the mouth of the Yangtze River.
This location brought Shanghai many advantages and allowed it to prosper and thrive and become and important part of the Asian part of the world, as well as the rest of it. In the past, and even now, it was very important for large cities to be situated near rivers and lakes and other types of waters so as to ensure the advancement of that city.

B.a pop up at ion changes in response to a change in the environment.