Density is defined as the ratio of the mass of the object to the volume of the object. Different substances have different densities. The mathematical expression for density is as follows: Density=Mass/Volume. The mass of a substance is expressed in kilograms and the volume is expressed in liters. The teacher did not think it was necessary to introduce 'density' to explain to them, because they form alternate conceptions about density as they are comparing many objects which would sink and float.they may develop many alternate conceptions and misconceptions regarding density.they may get confused the topic of density with the topic of weight.a new conception that is not true may be formed in children's minds.If the teacher introduces the concept of density also they may get confused by the concepts of mass and volume.Hence the teacher did not think it was necessary to introduce 'density' to explain to them, because they form alternate conceptions about density. Additional InformationAll the other options are not true as they don't relate to the motive of the teacher for not introducing the concept of density at that particular point of time and activity.