In the section titled, "Biological Adaptation," the author writes, "Natural selection is a mechanism (the only mechanism) known to cause the evolution of adaptations, so many biologists would define an adaptation as a characteristic that has evolved by natural selection." Which of the following accurately demonstrates this principle in action?

A
A pride of lions enters an isolated valley accessible only via a hidden mountain escarpment. There the lions find optimal conditions—minimal competition and access to shade, water, and prey in copious amounts. Nearly every lion in the pride survives. Over the span of a decade, their numbers more than double.

B
After vaccination campaigns throughout the 19th and 20th centuries, the WHO certifies the eradication of smallpox in 1979. Smallpox is one of two infectious diseases to have been eradicated, the other being Rinderpest (cattle plague), which was declared eradicated in 2011.

C
Breeding back—a process in which the deliberate selective breeding of domestic animals in an attempt to achieve an animal breed with a phenotype that resembles a wildtype ancestor—allows a targeted animal population, usually one that is extinct, to be artificially rekindled.

D
A blowfish exhibits an adaptation that enables it to fill its stomach with air, making it less attractive to predators. As a result, this blowfish survives and reproduces. Its offspring exhibit the same adaptation. They, too, survive and reproduce in greater numbers than others of their kind.

E
During winter, when weather conditions are harsh and resources are scarce, the American black bear enters the state of hibernation—a season of heterothermy characterized by low body temperature, slow breathing and heart rate, and low metabolic rate.