When doing medical research with human subjects, which four limitations are unavoidable?

While doing medical research on the human subject, following limitations are unavoidable:

a). Privacy: The privacy of the subject involved in the research should be maintained.

b). the subject must be allowed to the minimal exposure to risks and it must be tested on animals first to avoid deaths.

c). The risk associated with the physiological and physical well being of the subject must be considered.

d). The subject must be well instructed about all the risk and procedure involved in the research.

At the time of performing research with human subjects, there are limitations that can be unavoidable are as follows:  

a. Firstly there will be ethical concerns, that is, there are privacy or ethical concerns to look after.  

b. It is generally not possible to repeat trials on a similar subject.  

c. It can be tough to monitor all the probable variables.  

d. The subjects may report an imprecise medical past.  

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.