116. Read the following passage, then answer the question below.

Between roughly 800 and 950 CE, many of the great Maya cities of the southern lowlands were gradually abandoned. For more than a century, historians treated this 'collapse' as a mystery, and explanations ranged from warfare and disease to peasant revolt. Over the past few decades, however, evidence drawn from natural archives has changed the discussion. Stalagmites in Belizean caves, for example, record rainfall in their annual growth layers, and chemical analysis of these layers shows a series of severe droughts during the period of abandonment.

Drought alone, though, cannot explain why some cities collapsed while others survived. The Maya lived in political rivalries; kings built monuments and fought wars to display their power, and these ambitions depended on a steady surplus of maize. Cities in the southern lowlands relied on rain-fed fields and reservoirs, which gave them little protection when rains failed for years. Cities in the north, by contrast, were supplied by underground water sources and continued to flourish. A crisis of food, then, seems to have exposed the weakness of a political system whose legitimacy rested on its ability to guarantee prosperity. When harvests failed, ordinary people could have lost faith in rulers who claimed divine control over the rain.

Most scholars now avoid the word 'collapse', which implies a sudden and total disappearance. Millions of Maya remained in the region, and their descendants live there today. What ended was the system of royal courts in particular cities, not a civilization. The distinction matters, because it replaces a story of vanished people with one of communities that adapted to environmental stress by reorganizing the way they governed themselves.

The word 'which' in the second paragraph (reservoirs, which gave them little protection) refers to