Researchers Are Attempting to Conduct an Intervention Providing Public Transportation

Researchers are attempting to conduct an intervention providing public transportation vouchers to residents as a method of reducing neighborhood crime rates.In hindsight,researchers discovered that participants in the treatment group were already less likely to have a criminal record than the general population to which they were compared.This problem is an example of

A) selection bias.
B) matching procedure.
C) treatment condition.
D) control condition.