Aerobic organisms first started building up oxygen in the Earth's atmosphere around
A) 4.6 billion years ago
B) 3.5 billion years ago
C) 4.0 billion years ago
D) 2.5 billion years ago
D
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In a semiconductor doped with donor atoms, the conduction-band energy is 1.1 eV and the donor energy level is 1.0 eV. At a high temperature there are approximately as many electrons in the conduction band as there are holes in the valence band. For this semiconductor, the Fermi energy is approximately equal to:
(a) 1.1 eV (b) 1.0 eV (c) 1.05 eV (d) 0.55 eV
Starting from rest, a car accelerates down a straight road with constant acceleration a for a time t, then the direction of the acceleration is reversed, i.e., it is -a, and the car comes to a stop in an additional time t, the time for the whole trip being 2t. At what time, or times, is the average velocity of the car for the trip equal to its instantaneous velocity during the trip?
a. There is no such time. b. It is at the halfway point at t. c. This occurs at 2 times, at 0.5 t and 1.5 t. d. This occurs at 2 times, at 0.707 t and 1.293 t.
The Crab nebula is
a. a planetary nebula. b. a Bok globule. c. an open cluster. d. an absorption nebula. e. supernova remnant.
During an annular eclipse of the Sun, the corona of the Sun is visible
a. True b. False Indicate whether the statement is true or false