Astronomers believe that dark matter exists because
A. the galaxies in clusters move slower than expected on the basis of the material visible in them.
B. they can detect it with radio telescopes.
C. the outer parts of galaxies rotate faster than expected on the basis of the material visible in them.
D. it is the only way to explain the black holes in active galaxies.
E. All of these choices are correct.
Answer: A
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Why do scientists think that our solar system must have formed sometime after nearby supernovae explosions?
A) existence of heavy elements B) solar temperature too low C) our Sun is a G-type star D) They don't—scientists believe our Sun is among the first generation of stars.
The ________ habitable zone is set by the activity of the nucleus on the inside, and the rate of star formation and proper chemistry on the outside
Fill in the blank(s) with correct word
The basic function of the electric generator is which of the following conversion processes?
a. alternating current to direct c. low voltage to high or vice versa b. electrical energy to mechanical d. mechanical energy to electrical
An elevator suspended by a vertical cable is moving downward but slowing down. The tension in the cable must be
A) equal to the weight of the elevator. B) less than than the weight of the elevator. C) greater than the weight of the elevator.