Adouble-blind study of a vaccine is one in which : the study group receives the vaccine and the control group receives a placebo. neither observer nor subjects know the nature of the placebo. neither observer nor subjects know which subject receives the vaccine and which receives a placebo. neither the study group nor the control group knows the identity of the observers. the control group does not know the identity of the study group.

Neither observer nor subjects know which subject receives the vaccine and which receives a placebo

Explanation:

In a double-blind study neither the participants nor the experimenters is aware of who is receiving a particular treatment.

Its used to prevent bias in research results from demand characteristics or the placebo effect

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.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).