Which of the following statements best describes the origin of the Great Wall of China?（ ）

A、The Great Wall was built primarily during the Ming Dynasty to protect against invasions from the Mongols.
B、The Great Wall originated in ancient times as a series of small walls built by different states for defense purposes.
C、The Great Wall is solely a modern reconstruction and does not have historical significance.
D、The Great Wall was built exclusively for agricultural purposes.