A researcher is studying ways to improve the cognitions of severely depressed patients. She identifies one severely depressed patient and decides to administer a new depression treatment to the patient. After she administers the treatment, she measures the patient's cognitions to establish the patient's cognitive functioning. This subsequent measurement is part of the _______________ phase of a single-subject experimental design.
A) Pre-treatment steady state.
B) Baseline treatment.
C) Intervention.
D) Follow-up.