Viscosity is a measure of a fluid's resistance to flow, and gases typically have lower molecular interactions and are less dense than liquids, resulting in lower viscosity.
The presence of insoluble impurities in a liquid can decrease its surface tension. Surface tension is related to the cohesive forces between molecules at the surface of a liquid. Insoluble impurities can disrupt these cohesive forces, leading to a decrease in surface tension.
Hence, this is the correct option.