What is the significance of stromatolites?
A) They appear to be fossils of structures made by microbes long ago.
B) They indicate that microbes in the distant past were much larger than bacteria of today.
C) They show that the oxygen content of Earth's atmosphere was much smaller in the past.
D) They are the earliest known fossils of animals.
A) They appear to be fossils of structures made by microbes long ago.
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The atmosphere of Venus is very dry
a. True b. False Indicate whether the statement is true or false
The greater the temperature difference between input and output reservoirs of a heat engine, the
A) greater the efficiency. B) smaller the efficiency. C) both of the above D) none of the above
What compelling evidence links pulsars to neutron stars?
A) Both pulsars and neutron stars can be found in globular star clusters. B) Pulsars are known to evolve into neutron stars. C) Only a small, very dense source could rotate that rapidly without flying apart. D) Pulsars are always found in binary systems with neutron stars. E) Both pulsars and neutron stars have been discovered near the Sun.
Six months of darkness at the North Pole begins when the Sun is at the
a. winter solstice. b. summer solstice. c. autumnal equinox. d. vernal equinox.