The question was asked in a national level competition.
The question is from Classification of Defects & Factors Affecting Salvage of Castings in chapter Salvage of Defective Castings of Casting-Forming-Welding I
Easiest explanation: Sand inclusion is one of the most frequent causes for rejection of casting. It is very difficult to diagnose, as this type of defect normally occurs at varying positions in the casting which is caused by eroding of sand by a metal stream and then float to the surface of the casting. So it is not easy to attribute to a local cause.