What does the delta symbol represents in this equation

Which of the following statements about imprisonment rates in the united states is true? the united states imprisons people at a higher rate than russia, mexico, or canada. the united states imprisons people at a lower rate than russia or south africa, but higher than israel. the united states imprisons people at a lower rate than russia, mexico, or canada. russia, south africa, and mexico each imprison more people than the united states.