instruction:
You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
question:
Category: LANDMARKS 
Clue: Some of its pieces, which weighed up to 50 tons, were quarried at Marlborough Downs, about 20 miles away
answer:
stonehenge


question:
Category: U.S. PRESIDENTS 
Clue: He was the last man elected president who had served as a U.S. Senator
answer:
richard m. nixon


question:
Category: PLAYS 
Clue: The title of a Sean O'Casey play refers to these symbols on the flag of the Irish Citizen Army
answer:

the plough and the stars