What is the main reason blood flow is slowest in the capillaries?

Naturally acquired active immunity occurs when the body produces antibodies in response to an infection or vaccine. This is not the case for antibodies passed through the placenta.

Choice B rationale:

Artificially acquired passive immunity involves the injection of antibodies, not the transfer through the placenta.

Choice C rationale:

Innate immunity refers to nonspecific defense mechanisms that come into play immediately or within hours of an antigen’s appearance in the body. This does not describe the process of antibody transfer through the placenta.

Choice D rationale:

Natural passive immunity occurs when antibodies are passed from the mother to the fetus through the placenta. This matches the description in the question.

So, the correct answer is Choice D, after analyzing all choices.

Choice A rationale:

Veins do not always carry deoxygenated blood. The pulmonary veins are an exception as they carry oxygenated blood from the lungs to the heart.

Choice B rationale:

Veins do not carry blood away from the heart. They carry blood back toward the heart.

Choice C rationale:

Veins do not have a thick tunica media. The tunica media is thinner in veins than in arteries.

Choice D rationale:

Veins have valves. This is a true statement. The valves in veins prevent blood from flowing backward.