In recent years, cities around the world have been facing serious environmental challenges. Rapid urbanization has increased the demand for transportation, housing, and energy. As a result, air pollution, traffic congestion, and waste management problems have become more common in many metropolitan areas. One solution that many governments are exploring is the development of sustainable cities. A sustainable city is designed to reduce environmental damage while improving the quality of life for its residents. This can include promoting public transportation, building energy-efficient homes, increasing green spaces, and encouraging recycling. Public transportation plays a crucial role in reducing the number of private vehicles on the road. When people use buses, trains, or metro systems instead of cars, fuel consumption decreases and air pollution is reduced. In addition, modern cities are increasingly investing in cycling lanes and pedestrian-friendly infrastructure to encourage healthier and more environmentally friendly modes of travel. Another important aspect of sustainability is efficient energy use. Many cities are adopting renewable energy sources such as solar and wind power. Smart technologies are also being used to monitor energy consumption and reduce waste. For example, smart streetlights automatically adjust brightness depending on the amount of natural light available.However, creating sustainable cities requires cooperation between governments, businesses, and citizens. While policies and infrastructure are important, individual behavior also plays a major role. Simple actions such as using public transport, conserving electricity, and reducing waste can collectively make a significant difference.According to the passage, who is responsible for building sustainable cities?