Upon Deciding to Study Individuals with a Given Disorder, What

Upon deciding to study individuals with a given disorder, what is the next step that should be taken?

A) Determine what treatment approach will be tested.
B) Select criteria for identifying individuals with the disorder.
C) Gather survey data to determine where your subjects are most likely to reside.
D) Establish which subjects will be the control group and which will be in the experimental group.