The mass extinction at the end of the Cretaceous period was likely the result of:
a. rising seawater.
b. loss of vegetation.
c. extreme predation.
d. a dramatic change in climate.
e. the appearance of new competitors.
d
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What does it mean when we say a gene is "turned off"?
A. The gene cannot be transcribed and translated into a protein. B. The gene is no longer working properly. C. The gene is now activated. D. The gene has a mutation.
In the trp operon, __________ in the absence of tryptophan.
a) all of the choices are correct b) the trp operon structural genes are transcribed c) the operator site is open to allow RNA polymerase binding d) the tryptophan-synthesizing enzymes of the trp operon are synthesized
The release of most nitrogen oxides comes from ____.
A. ammonia-oxidizing bacteria B. the use of DDT C. coal-burning power plants D. drilling for gasoline and oil E. combustion of gasoline and oil
The movement of substances from higher to lower concentration across a semipermeable membrane that must have a specific protein carrier but no energy expenditure is called
A. active transport. B. diffusion. C. facilitated diffusion. D. endocytosis. E. osmosis.