Blood flow through a capillary is controlled by a precapillary sphincter, which opens or closes due to the requirements of the cells supplied by the capillary.

It explains that the primary immune response produces antibodies within five to ten days of exposure to antigens, while the secondary immune response produces antibodies within a day or two of subsequent exposure to the same antigen.
This is because the primary immune response involves the activation and differentiation of naive B cells into plasma cells and memory cells, while the secondary immune response involves the rapid activation of memory cells into plasma cells.
Choice A is wrong because the primary and secondary immune responses can occur in both children and adults.
Choice B is wrong because the primary immune response is slower than the secondary immune response in producing antibodies.
Choice D is wrong because both primary and secondary immune responses are important for the body’s ability to protect itself from pathogens.
The primary immune response generates immunological memory, while the secondary immune response provides a faster and stronger response to repeated infections.

The systemic circuit sends oxygen-rich blood to the tissues.

It is part of the circulatory system that carries blood away from the heart, delivers it to most of the organs and tissues, and returns it to the heart again.
Choice A is wrong because it describes the pulmonary circuit, which brings oxygen-poor blood from the heart to the lungs.
Choice B is wrong because it is the opposite of what the systemic circuit does.
The systemic circuit brings oxygen-rich blood from the heart to the tissues, not from the tissues.
Choice C is wrong because it is also the opposite of what the systemic circuit does. The systemic circuit sends oxygen-poor blood to the heart, not from the heart.