In a Standard Roy Model, Migration Can Lead to an Increase

In a standard Roy model, migration can lead to an increase in average skills in both locations. What is necessary for this to happen?

A) The workers who migrate from the source country are below-average in skill relative to the average source country person but are above-average in skill relative to the average destination country person.
B) The workers who migrate from the source country are above-average in skill relative to the average source country person but are below-average in skill relative to the average destination country person.
C) The highest skilled workers migrate from the source country to the destination country.
D) The lowest skilled workers migrate from the source country to the destination country.
E) Migration leads to an increase in average skills in both locations whenever there is positive selection.