Which of the Following Techniques or Actions Represents Constancy in Data

Which of the following techniques or actions represents constancy in data collection for control of extraneous variables in a study examining the effect of hormone replacement therapy on bone density loss in postmenopausal women?

A) Drawing blood from all participants at the same time of day
B) Always having the same nurse or phlebotomist always draw blood from all participants
C) Using the same room for drawing blood from all participants
D) Ensuring that all participants are postmenopausal women taking hormone replacement therapy