Tasha is a graduate student studying health and nutrition. A recent study has made claims about the health benefits of dark chocolate. Tasha would like to conduct a similar study to see if she can confirm the findings of the first. She wants to make sure that her study is as close to the original as possible. Which factors of her study can she control? Check all that apply.

lifestyle and physical condition of the subjects
the number of subjects
genetic variances among the subjects
age and sex of the subjects
length of the study