In Order to Investigate Treatments for Morbid Obesity, Obese Subjects

In order to investigate treatments for morbid obesity, obese subjects satisfying fairly strict requirements were randomly assigned to one of three treatment groups: (1) gastric bypass surgery, (2) participation in a diet and exercise program, or (3) both gastric bypass surgery and participation in a diet and exercise program. Researchers observed the amount of weight lost five years after the study began. The response is:

A) the kind of program to which a subject was assigned.
B) the amount of weight lost five years after the study began.
C) gastric bypass surgery.
D) random assignment.