Chemistry
The major harmful gas emitted by automobile vehicles which causes air pollution is

Carbon Monoxide
Carbon dioxide
Methane
Ozone gas

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Chemistry
Water is not effective in extinguishing a fire caused by petrol because

The flame is too hot for water to cool it down
Water and petrol react chemically
Water and petrol are miscible with each other
Water and petrol are immiscible with each other and petrol which forms the upper layer continues to burn

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