Which of the following is classified as a nominal variable?

Correct Answer: A

Rationale: Nominal variables are categorical without order. 'Ethnicity,' is correct, as it labels groups (e.g., Asian, Caucasian) without ranking, per statistical classification (e.g., Agresti). 'Socio-economic status,' is ordinal, with levels (e.g., low, high). 'Class positions,' is ordinal, implying rank. 'Weight,' is ratio, measurable with a zero. Ethnicity's lack of numerical or hierarchical properties makes A the precise nominal answer.

Correct Answer: D

Rationale: Antigen-presenting cells (APCs) process and display antigens to T-cells, initiating adaptive immunity. 'Natural killer cells,' is correct as the exception, per immunology references like Abbas' Basic Immunology, as NK cells target infected cells directly, not presenting antigens. 'Macrophages,' engulf pathogens and present antigens via MHC. 'Dendritic cells,' are potent APCs, activating naive T-cells. 'B-Lymphocytes,' present antigens to helper T-cells for antibody production. NK cells' innate cytotoxic role, lacking MHC presentation, distinguishes D as the accurate answer, separate from APC functions.

Correct Answer: B

Rationale: The uterine (fallopian) tube is about 10 cm long, extending from ovary to uterus. '10 cm,' is correct, per anatomy texts like Netter's Atlas, varying slightly (7-12 cm) but typically cited as 10 cm. '5 cm,' is too short. '15 cm,' and '20 cm,' exceed average length. The tube's role in ovum transport supports B's accuracy, reflecting standard anatomical measurements.

Correct Answer: C

Rationale: Essential amino acids must be dietary, as the body cannot synthesize them. 'Alanine,' is correct as the exception, a non-essential amino acid produced from pyruvate, per biochemistry (e.g., Nelson). 'Leucine,' 'Lysine,' and 'Valine,' are essential, required from food. Alanine's endogenous synthesis contrasts with the dietary need for others, making C the precise answer.

Correct Answer: C

Rationale: Villi are the small intestine's functional units, maximizing absorption. 'Villi,' is correct, per histology (e.g., Ross), with enterocytes for digestion and uptake. 'Mucosa,' is a layer containing villi. 'Muscularis,' aids motility. 'Serosa,' is the outer layer. Villi's absorptive role makes C the precise answer.