19. The switch of one B cell from producing one class of antibody to another antibody class that is responsive to the same antigen is due to
(A) the rearrangement of immunoglobulin heavy-chain C region DNA.
(B) a switch in the kind of antigen-presenting cell that is involved in the immune response.
(C) a patient's reaction to the first kind of antibody made by the plasma cells.
(D) the rearrangement of V region genes in that clone of responsive B cells.

15. Which of the following habitats would most likely have a high proportion of species of plants that are in symbiotic association with nitrogen-fixing bacteria? (A) anoxic soils such as fens and bogs (B) agriculturally productive regions that have gone fallow (C) nutrient-limiting soils such as wet tropical rain forests (D) alpine coniferous forests

16. Which of the following statements regarding flowering plants is false? (A) Pollination is the placing of pollen on the stigma of a carpel. (B) The food-storing endosperm is derived from the cell that contains two polar nuclei and one sperm nucleus. (C) Flowers produce fruits within the ovules. (D) Female gametophytes develop from megaspores within the anthers.

17. Which of the following hormones would be most useful in promoting the rooting of plant cuttings? (A) auxins (B) gibberellins (C) abscisic acid (D) oligosaccharins

18. The velocity of blood flow is the lowest in capillaries because (A) the capillary walls are not thin enough to allow oxygen to exchange with the cells. (B) the total cross-sectional area of the capillaries is greater than the total cross-sectional area of the arteries or any other part of the circulatory system. (C) the capillaries are far from the heart, and blood flow slows as distance from the heart increases. (D) the diastolic blood pressure is too low to deliver blood to the capillaries at a high flow rale.

20. Osmoregulatory adjustment via the renin-angiotensin-aldosterone system can be triggered by (A) drinking several glasses of water. (B) eating a pizza with olives and pepperoni. (C) sleeping for one hour. (D) severe sweating on a hot day.

21. Asexual reproduction results in greater reproductive success than does sexual reproduction when (A) pathogens are rapidly diversifying. (B) a species is in stable and favorable environments. (C) a species has accumulated numerous deleterious mutations. (D) a species is expanding into diverse geographic settings.

22. The lateral line system in fish transduces sensory information in a manner that, among these choices, is most similar to (A) human olfaction. (B) human vestibular sense‘ (C) human vision. (D) human thermoreception,

23. "How do seed-eating animals affect the distribution and abundance of the trees?" This question (A) would require an elaborate experimental design to answer. (B) would be difficult to answer because a large experimental area would be required. (C) would be difficult to answer because a long-term experiment would be required. (D) All options are correct.

24. Which of the following causes populations to shift most quickly from an exponential to a logistic population growth? (A) competition for resources (B) increased birth rate (C) removal of predators (D) favorable climatic conditions

25. Which of the following studies would shed light on the mechanism of spread of H5N1 from Asia? (A) Test fecal samples for H5N1 in Asian waterfowl that live near domestic poultry farms in Asia. (B) Test for the presence of H5N1 in poultry used for human consumption worldwide. (C) Perform cloacal or saliva smears of migrating waterfowl to monitor whether any infected birds show up in Alaska. (D) Keep domestic and wild fowl from interacting with each other to minimize the probability that wild fowl could get infected and migrate out of Asia.