A list should be introduced with a colon when the list is preceded by:

The correct answer is B. a complete thought.

A colon is typically used before a list when the sentence leading up to it is a complete thought or independent clause. This informs the reader that what follows is directly related to the statement made before the colon. For example: “You need the following items: a pen, paper, and a calculator.” Here, “You need the following items” is a complete thought.

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