The Namibian cheetah (Acinonyx jubatus jubatus), the world's fastest land-dwelling animal, hunts across the African grasslands using high-speed sprints.  Cheetahs possess unique anatomical features specialized for speed, including an aerodynamic skull, elongated legs, and an enlarged heart.  These physiological adaptations have occurred over time and allow cheetahs to achieve speeds greater than 100 km/h.  However, due to their small body and slender limbs, cheetahs (50–64 kg) are limited with regard to the size of the animals they can successfully hunt and therefore typically prey on slightly smaller species that inhabit open grasslands.It is believed that around 12,000 years ago, Namibian cheetahs experienced an environmental catastrophe that drastically reduced their population.  The small number of surviving cheetahs began mating with one another, which led to severe inbreeding depression.  Over time, the offspring of subsequent generations exhibited decreased fitness, reduced fecundity (number of offspring), and sustained a loss of major histocompatibility complex (MHC) allele diversity.  To study if low MHC diversity correlates with low fitness in cheetahs, scientists analyzed the genetic diversity and disease prevalence in a group of free-ranging and captive male cheetahs.Experiment 1MHC allele genotyping and analysis was performed in wild (free-ranging) and captive male cheetahs (Figure 1).  Because MHC diversity is strongly correlated with genetic diversity, MHC diversity is used as a marker for an organism's overall genetic variability.Figure 1  Evaluation of heterozygosity in (A) wild and (B) captive male cheetahs born between 1976 and 2007 (Note:  Each data point represents a single male cheetah.)Experiment 2Disease prevalence was evaluated in wild and captive male cheetahs by quantitative analysis of symptoms relating gastrointestinal, liver, and kidney disease.  The percentages of cheetahs affected by each disease are shown in Figure 2.Figure 2  Evaluation of disease prevalence in wild and captive male cheetahsExperiment 3Fecal cortisol levels were noninvasively evaluated in wild and captive cheetahs over a period of 6 months (Figure 3).Figure 3  Average fecal cortisol concentrations of wild and captive male cheetahsAdapted from Castro-prieto A, Wachter B, Sommer S. Cheetah paradigm revisited: MHC diversity in the world's largest free-ranging population. Mol Biol Evol. 2011;28(4):1455-68. Question 35Researchers concluded that captive cheetahs have reduced fitness because of their increased susceptibility to diseases.  What can best explain the decreased fitness of captive cheetahs described in the passage?A.Captive cheetahs have increased MHC diversity.B.Captive cheetahs have decreased MHC diversity.C.Captive cheetahs experience increased environmental stress.D.Captive cheetahs experience decreased environmental stress.