Answer:
It’s often impossible to repeat trials on the same subjects.
Subjects may report an inaccurate medical history.
It can be difficult to control all possible variables
There are ethical or privacy concerns to consider.
Explanation:
a) Accuracy of medical history - A person on which the research is being conducted may or may not report an accurate medical history.
b) Repeating Trials on subject - Before any medical research, the subject is made aware of risk associated with the research. And only after the consent of the subject research is done. In such scenarios, it becomes very difficult to repeat the trials.
c) It is very difficult to control all the variables in a research being conducted on living being
d) Social security concerns lies with the subject of the research. Without his/her concern, the result of the research cannot be revealed but usually for study purpose the data is shared sometimes with the scientist.
Hence, option A, B, C and E are correct