In October 2017, Scientists Presented Data at the American Society

In October 2017, scientists presented data at the American Society of Anesthesiologists meeting in Boston, MA which suggested that Botox, a drug derived from the neurotoxin produced by the bacterium Clostridium botulium, can reduce the number of migraines and their duration as well as the amount of pain experienced during migraine episodes in children and teenagers. The study was completed with nine children between the ages of 8 and 17. Before the study began, the study participants reported having migraines anywhere from 8 and 29.5 days each month, and these episodes lasted 30 minutes-24 hours each. During the study, all participants received Botox injections every 12 weeks for five years; injections were given in both the front and back of the participants' heads and necks. At the end of the five-year study, participants reported suffering from migraines between 2 and 10 days per month, and these episodes lasted 15 minutes-7 hours. Intensity of migraine pain also decreased from pre-study scores between 4 and 8 on a scale of 1-10 (1 being no pain and 10 being terrible pain) to scores of 1.75 to 5 post-study.
As described, what experimental design factor is missing in the Botox study?

A) an experimental group
B) a control group
C) independent variable
D) dependent variable