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.

Uterine contractions during childbirth illustrate a positive feedback mechanism.

A positive feedback mechanism is a process in which the end products of an action cause more of that action to occur in a feedback loop.
This amplifies the original action.
For example, when a woman goes into labor, the pressure of the baby’s head on the cervix stimulates nerve impulses that travel to the brain and trigger the release of oxytocin, a hormone that causes the uterus to contract.
The contractions increase the pressure on the cervix, which stimulates more nerve impulses, more oxytocin, and more contractions.
This cycle continues until the baby is delivered.
Choice A is wrong because body temperature control is an example of a negative feedback mechanism, which is when the end results of an action inhibit that action from continuing to occur.
For example, when the body temperature rises above normal, the skin sweats, and blood vessels dilate to release heat.
This lowers the body temperature back to normal and stops sweating and dilation.

Choice B is wrong because control of blood sugar is also an example of a negative feedback mechanism.
For example, when the blood sugar level rises after a meal, the pancreas secretes insulin, a hormone that helps cells take up glucose from the blood.
This lowers the blood sugar level back to normal and stops the insulin secretion.
Choice D is wrong because maintaining blood pressure is another example of a negative feedback mechanism.
For example, when the blood pressure drops due to blood loss or dehydration, the heart beats faster and stronger, and the blood vessels constrict to increase the blood pressure.
This restores the blood pressure back to normal and stops the heart rate and vessel constriction.