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The title of Prince Harry's memoir, "Spare," is inspired by a phrase commonly used in the context of royal succession. What does the phrase "the spare" traditionally refer to in British royal history?
A) The monarch's primary heir to the throne.
B) A sibling of the monarch who is not the heir apparent.
C) A royal adviser who assists with state affairs.
D) A historical figure who was once a contender for the throne.