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.

Naturally acquired active immunity is the type of resistance that is acquired as a result of developing a disease.
This means that the immune system produces antibodies to fight off the infection and remembers the pathogen for future protection.
This type of immunity is long-lasting and sometimes life-long.
Choice A is wrong because naturally acquired passive immunity is the type of resistance that is acquired when a person receives antibodies from another source, such as from the mother through the placenta or breast milk.
This type of immunity is temporary and lasts only for a few weeks or months.
Choice C is wrong because artificially acquired active immunity is the type of resistance that is acquired when a person receives a vaccine that contains a weakened or killed form of the disease organism.
This triggers the immune system to produce antibodies and memory cells without causing the actual disease.
This type of immunity can last for years or decades, depending on the vaccine.
Choice D is wrong because artificially acquired passive immunity is the type of resistance that is acquired when a person receives antibody-containing blood products, such as immune globulin, that provide immediate protection from a specific disease.
This type of immunity is also temporary and lasts only for a few weeks or months.

Macrophages are the main cells that clear out the cell debris by phagocytosis, a process that involves recognition, engulfment, and degradation of the disposable particles.

Macrophages are professional phagocytes that can be found in various tissues and organs, where they perform efferocytosis, the removal of dead and dying cells.

Choice A is wrong because lymphocytes are not phagocytes, but rather immune cells that mediate adaptive immunity by producing antibodies or killing infected cells.

Choice B is wrong because cytokines are not cells, but rather soluble molecules that regulate inflammation and immunity by acting as signals between cells.

Choice C is wrong because mast cells are not primarily involved in clearing cell debris, but rather in allergic reactions and innate immunity by releasing histamine and other mediators.