The loose-fitting sac around the heart is the:

Innate defenses counter specific disease-causing agents, whereas adaptive defenses include mechanical and chemical barriers.

Choice A is wrong because it confuses the two types of defenses.

Innate defenses are nonspecific and include physical barriers such as the skin, molecules that are toxic to invaders, and phagocytic cells that ingest invaders.

Adaptive defenses are specific and are activated by the innate immune system.

They involve the production of antibodies and specialized cells that recognize and eliminate specific pathogens.

Normal ranges are not applicable in this question as it is about the definitions of innate and adaptive defenses.

Circadian rhythm is the term for the biological clock that regulates the sleep-wake cycle and other physiological processes in the body.
Melatonin is a hormone that is produced by the pineal gland in the brain in response to darkness.
It helps with the timing of the circadian rhythm and promotes sleep.
Choice A is wrong because metabolic rate is not affected by light exposure, and thyroxine is a hormone that regulates metabolism, not sleep.
Choice B is wrong because the limbic system is a part of the brain that is involved in emotions, memory, and motivation, not sleep.
Thymosin is a hormone that stimulates the development of T cells, which are part of the immune system.
Choice C is wrong because the circulatory system is the system of blood vessels and organs that transport blood throughout the body, not a part of the brain that controls sleep.
Dopamine is a neurotransmitter that is involved in reward, motivation, and movement, not sleep.