Read the passage and answer: The Great Wall of China was built over many centuries by several dynasties. Construction began more than 2,000 years ago, and the wall was extended and rebuilt repeatedly. Its primary purpose was defense against invasions from the north. According to the passage, what was the wall's primary purpose?

This is a detail question, which asks for a fact stated directly in the passage. The text says the wall's 'primary purpose was defense against invasions from the north,' so the answer is defense against northern invasions. For detail questions, locate the specific statement in the passage rather than relying on outside knowledge or inference. The other choices are not supported by the passage. The correct answer restates the explicit fact: the wall was built primarily for defense against invaders from the north.