***** NOT A TEACHER *****
I am 100% sure that today we always refer to the United States of America as one nation. Therefore, your #18 is, indeed, correct.
For example, the Los Angeles Times, my local newspaper, is considered to be one of the best American newspapers.
Before our Civil War (1861 -1865), many people felt that this country was a voluntary union of independent states. So at that time, people did prefer to say "The United States are ...." After the Civil War, people started to feel that this was, in fact, one single nation. Therefore, people started to use "is" instead of "are."
Student or Learner