Read the following passage regarding the Classic Maya collapse.
[1] Historians have traditionally attributed the ninth-century collapse of the Classic Maya civilization either to internal political fragmentation or to catastrophic agricultural failures caused by overpopulation. [2] Recent paleoclimatological data, however, indicates that the region experienced a series of severe, century-long megadroughts precisely during the period of political disintegration. [3] While some scholars argue that this climate data establishes drought as the sole driver of the collapse, such environmental determinism overlooks the resilience of earlier Maya societies during similar dry periods. [4] Instead, the evidence suggests that the Classic Maya collapse was fundamentally the result of inflexible political structures that prevented elites from adapting traditional water management systems to unprecedented environmental stress. [5] By clinging to ideologically entrenched agricultural practices, the ruling class exacerbated the effects of the drought, ultimately fracturing their political legitimacy.
Based on the passage, which sentence explicitly formulates the author's main thesis? (Enter the sentence number only, e.g., '1', '2', etc.)
Answer: 4 / four / Sentence 4 / sentence 4 / 4.