Why should the sand used in the sand cone method be allowed to reach an air-dried state?

The correct choice emphasizes the importance of allowing sand to reach an air-dried state in order to ensure precise measurements during the sand cone method. When sand is in an air-dried state, it reaches a consistent moisture content, which removes variables related to moisture that could affect the accuracy of the measurements.

If the sand were damp or wet, it would lead to an increase in weight due to the presence of water, which could cause inaccuracies in the volume determinations made during the test. Moisture can also cause the sand to clump together, impacting the uniformity and flow of the sand during the filling process in the cone. Ensuring that the sand is air-dried helps achieve a consistent particle interaction and density, which is essential for obtaining reliable results in the determination of soil density and compaction.

The other choices present factors that might be encountered with sand usage but do not directly address the primary reason for air-drying, which is to enhance measurement accuracy.