21. In a comparison of birds and mammals, having four limbs is __________.
(A) a shared ancestral character
(B) a shared derived character
(C)a character useful for distinguishing birds from mammals
(D) an example of analogy rather than homology
(E)None of the above.

17. It has been observed that organisms on islands are different from, but closely related to, similar forms found on the nearest continent. This is taken as evidence that________. (A) island forms are descended from mainland forms (B) common environments are inhabited by the same organisms (C)island forms and mainland forms have identical gene pools (D) the island forms and mainland forms are converging (E)None of the above.

18. Genes in chromosomes packed into _______ fiber show the highest transcriptional activity. (A) 10 nm (B) 30 nm (C)supercoiling (D) metaphase chromosome (E)naked DNA

19. Which factor and mechanism regulates population growth in negative feedback manner typically so that population growth may follow the logistic growth model? (A) earth quakes； density-dependent (B) infectious disease； density-dependent (C)competition for resources； density-independent (D) toxic wastes； density-independent (E)toxic wastes； competition for resources

20. Given a population that contains genetic variation, what is the correct sequence of the following events under the influence of natural selection? 1. Well-adapted individuals leave more offspring than do poorly adapted individuals. 2. A change occurs in the environment. 3. Genetic frequencies within the population change. 4. Poorly adapted individuals have decreased survivorship. (A) 2→4→1→3 (B) 4→2→1→3 (C)4→2→3→1 (D) 2→4→3→1 (E)3→4→2→1

22. Which virus is responsible for causing infectious mononucleosis? (A) Epstein-Barr virus (B) Cytomegalovirus (C)Herpes simplex virus (D) Varicella-zoster virus (E)None of the above.

23. Which is NOT the factor contributed to prokaryotes genetic diversity? (A) Rapid reproduction (B) Genetic recombination (C)Mutation (D) Mutualism (E)All of the above.

24. What part of the bacterial cell helps it move? (A) Capsule (B) The flagella (C)Pili (D) Fimbriae (E)Cilia

25. The maternal factor _______ define the anterior/head structure of a fly embryo. (A) Torso (B) Bicoid (C)Nanos (D) Antennapedia (E)Dorsal

26. Which two joined amino sugars form a peptidoglycan monomer? (A) N-acetylglucosamine and N-acetylmuramic acid (B) N-acetylglucosamine and N-acetyltalosaminuronic (C)N-acetylmuramic acid and pseudomurein (D) Galactosamine and Daunosamine (E)None of the above.

27. Bacterial conjugation is an example of _______ .(A) Transduction (B) Vertical gene transfer (C)Horizontal gene transfer (D) Bacterial transformation (E)None of the above.