The best way to establish a causal relationship between taking aspirin and the risk of the aggressive form of prostate cancer is to conduct a randomized experiment in which researchers randomly choose who takes aspirin and who does not. For which of the following reasons might this not be possible in this case?

A. In a randomized experiment, the research staff should not know who is getting the treatment so they do not inadvertently influence the results. All of the research staff would know which participants were taking aspirin.
B. Since aspirin has been established as preventative for cardiovascular issues, ethical issues may prevent researchers from randomly denying some participants the opportunity to take aspirin.
C. It is very difficult to randomly decide who should take aspirin and who should not.
D. In a randomized experiment, participants should not know if they are getting the treatment or not. Participants would know if they were taking aspirin.