In a Study to Help People Quit Smoking Cigarettes,a Group

In a study to help people quit smoking cigarettes,a group of participants received one supportive phone call 10 days after attending a program on strategies to resist smoking.A second group received a supportive phone call every week for 6 weeks after attending the program,and a third group received no supportive phone calls after attending the program.What property of experimental research did the researcher employ in this study?

A) Quasi-experimental research, no control group
B) Random assignment to research groups
C) Manipulation of the intervention dose
D) Controlling for extraneous variables