Read the passage. The Saint Lawrence Seaway was built to allow oceangoing vessels to reach the Great Lakes. It was seen as a great boon for trade. However, it has proven to be extremely detrimental to the environment. After the seaway was built, a number of invasive species arrived on ships and began to plague the area. Residents along the river have also experienced dangerously high water levels because of the seaway. Which text structure is used in this passage?