Alkali metals (lithium, sodium, potassium) are so soft that they can be cut with a knife. The freshly cut surface is a shiny, silver colour, but this tarnishes quickly to a dull grey as the metal reacts with oxygen and water in the air.

Answer: A) Sulphur dioxide Explanation: Air pollutants are solid particles, gases, and liquid droplets in the air that can adversely affect ecosystems and the health of humans.   Major ambient air pollutants include toxic metals, polycyclic aromatic hydrocarbons, benzene, particulate matter (PM), nitrogen oxides, sulfur oxides, carbon monoxide, and ozone. Power plants, factories, and vehicle and air traffic are the most important anthropogenic sources of air pollution.   In addition, natural sources such as wildfires and volcanic activity contribute to ambient air pollution.