20. Which of the following best explains why ocean acidification, driven by increasing atmospheric CO2 from global warming, poses a major threat to organisms that build shells or skeletons from calcium carbonate (CaCO3)? (A) Rising sea temperatures directly dissolve existing CaCO3 structures in corals and mollusks. (B) Increased CO2 in seawater reacts to form carbonic acid, lowering pH and decreasing the concentration of carbonate ions (CO3 2−), making it harder for marine organisms to precipitate CaCO3. (C) Global warming reduces ocean salinity, preventing calcium ions from binding effectively with carbonate ions. (D) Ocean acidification increases the availability of carbonate ions, causing excessive and uncontrolled CaCO3 deposition that smothers organisms. (E) Higher CO2 levels promote photosynthesis in calcifying algae, depleting calcium ions from seawater and starving shell-forming animals.

4. Which of the following descriptions is incorrect for a recognized type of natural selection? (A) Directional selection: A mode of natural selection in which individuals with phenotypes at one extreme of a trait distribution have higher fitness than those with intermediate or opposite extreme phenotypes. (B) Stabilizing selection: A mode of natural selection that favors both phenotypic extremes over intermediates, potentially increasing variation and leading to a bimodal distribution or speciation. (C) Balancing selection: A mode of natural selection that maintains genetic diversity in a population by favoring multiple alleles, often through mechanisms like heterozygote advantage or varying selection pressures across environments. (D) Purifying selection: A mode of selection that removes deleterious mutations or alleles, preserving functional genetic sequences and reducing genetic variation at conserved loci. (E) Frequency-dependent selection: A mode of selection where a phenotype's fitness depends on its frequency in the population—often, rare phenotypes have higher fitness.

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