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

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