Concept:Surface tension of a liquid is caused by cohesive forces between its molecules, and it is reduced when these forces are weakened.
Explanation:Detergents are surfactants, meaning they actively lower the surface tension of water.
Each detergent molecule has a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail.
When detergent is added to water, these molecules gather at the surface.
They insert themselves between water molecules and interfere with the hydrogen bonding in
H2O.
This disruption weakens the net inward pull on the surface molecules, so the surface tension decreases.
Metal, sand, and paper do not have this surfactant property and do not significantly reduce the surface tension of water.
Therefore, the substance that lowers the surface tension of water is detergent.
Answer:C. detergent