What is the two-letter postal abbreviation for South Dakota?

In what year did South Carolina ratify the U.S. Constitution? What is the official nickname of South Carolina? What is the state bird of South Carolina? What is the state flower of South Carolina? Which state uses the postal abbreviation "SD"? Pierre is the capital of which state? What is the largest city in South Dakota by population? In what year was South Dakota admitted to the Union as a state?