Can you beat this one?

Which choice best states the main idea of the text?

1. AMinerals found in one Tikal reservoir suggest that the city's inhabitants developed an early method for filtering stored drinking water.
2. BResearchers have determined that most of Tikal's reservoirs were constructed using minerals transported from sources outside the city.
3. CGeological evidence indicates that the Corriental reservoir received naturally filtered water from a distant underground source.
4. DAnalysis of Tikal's reservoirs suggests that their builders selected different filtration materials according to the minerals available nearby.