Why should the pka of a leaving groups conjugate acid be considered when performing a substitution reaction

 A : pKa values have little effect on a functional group's ability to leave. There is no need to consider this when outlining a substitution reaction. B : In order to have a substitution reaction a compound needs to a have a functional group that will serve as a good leaving group. A high pKa (pKa > 5) of a conjugate acid produces the weakest conjugate bases in a substitution reaction. C : In order to have a substitution reaction a compound needs to a have a functional group that will serve as a poor leaving group. A low pKa (pKa > 0) of a conjugate acid produces the weakest conjugate bases in a substitution reaction. D : In order to have a substitution reaction a compound needs to a have a functional group that will serve as a good leaving group. A low pKa (pKa < 0) of a conjugate acid produces the weakest conjugate bases in a substitution reaction. This favors a substitution reaction.