A Study Is Examining the Effect of a New Antilipemic

A study is examining the effect of a new antilipemic drug,compared with placebo,on lipoprotein levels in postmenopausal women.Which of these actions represents the researcher's attempt at constancy in data collection to control extraneous variables?

A) Drawing the serum lipoprotein levels of all participants at the same time of day
B) Having the same phlebotomist draw serum lipoprotein levels of all participants
C) Using the same room for drawing blood for lipoprotein levels of all participants
D) Ensuring that all participants are postmenopausal women taking lipid-lowering medication