What Is "Brain Drain" in the Context of School Choice

What is "brain drain" in the context of school choice?

A) Families tend to choose less academically challenging schools so that their students earn higher grades, but are not scholastically challenged.
B) Talented students drain resources from schools such that low-performing students are not adequately educated and cared for.
C) Academically talented students leave low-performing schools in search of better opportunities, thus leaving the school worse off.
D) So many students are using vouchers that there are long waiting lists and over time, families lose patience and move out of state.