Reading Comprehension — quick quiz

Read the passage carefully and answer the question that follows: The phenomenon of urbanization in developing countries presents both opportunities and challenges. Rapid urban growth has led to the concentration of economic activities, creating employment opportunities and fostering innovation. Cities serve as engines of economic growth, contributing disproportionately to national GDP. However, unplanned urbanization has resulted in the proliferation of slums, inadequate infrastructure, traffic congestion, and environmental degradation. Water and sanitation facilities often fail to keep pace with population growth, leading to health hazards. Moreover, the rural-urban divide has widened, with rural areas experiencing depopulation and loss of productive workforce. Sustainable urban planning that integrates housing, transportation, and basic services is essential to harness the benefits of urbanization while mitigating its adverse effects. Policymakers must balance economic development with environmental sustainability and social equity to create livable cities for all residents. Question: What does the phrase 'engines of economic growth' in the context of the passage most nearly mean?