instruction:
Given an trivia question precisely answer the question with a word/phrase/name. External resources such as Wikipedia could be used to obtain the facts.
question:
Graca Machel, the widow of a President of Mozambique, is the wife of which famous man?
answer:
nelson mandela


question:
What is the SI Unit of Luminous Intensity?
answer:
candela


question:
Who directed the 2001 film `Harry Potter and the Philosopher`s Stone`?
answer:

chris columbus