The absence of atmospheric oxygen probably made life possible because oxygen ________
a. forms free radicals that degrade compounds by extracting electrons from their bonds
b. would have favored the development of early predators and suppressed life's first efforts
c. is a greenhouse gas that would have made the Earth too hot for life
d. prevents the formation of liquid water
a
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Protists cause four of the following diseases. Which one is the exception?
a. dysentery b. neurotoxic shellfish poisoning c. malaria d. AIDS e. trichomonas infections of the reproductive tract
Of all primates, which are genetically the closest to humans?
a. the great apes b. spider monkeys c. gibbons d. bonobos e. baboons
Life probably would never have emerged on Earth if the ancient atmosphere had been the same as the present one in regard to the content of ____.
A. carbon dioxide B. hydrogen C. oxygen D. nitrogen E. carbon monoxide
Single-celled organisms called ciliates are
A) fungi. B) protozoa. C) insects. D) algae. E) water molds.