What was the immediate cause of the end-Cretaceous extinction event?
a. a gamma-ray burst
b. rising sea levels
c. an asteroid impact
d. increased sunspot activity
c. an asteroid impact
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The electrochemical gradient that determines how a charged ion will move through a permeable membrane depends upon
A. the electrical forces acting on that ion. B. the ratio between the charge on the ion and its atomic mass. C. the combined effects of electrical forces and concentration differences across the membrane. D. the difference in concentration of that ion on either side of the membrane. E. All of the answers are correct.
Hemoglobin is composed of four _______.
A. protein chains B. heme groups C. iron atoms D. two of the above E. all of the above
What is the role of helper T cells in the adaptive immune response?
a. Helper T cells directly kill infected host cells. b. Helper T cells activate B cells and cytotoxic T lymphocytes to kill infected host cells. c. Helper T cells phagocytize bacteria and viruses. d. Helper T cells produce and secrete antibodies.
Which of the following is not one of the stages of HIV infection?
A. flat or raised lesions on the penis or vulva B. opportunistic infections C. a CD4 + T cell count between 200 and 499 per microliter of blood D. a greater than 500 CD4 + T cells per microliter of blood E. a CD4 + T cell count below 200 per microliter of blood