What event made it possible for photons to begin to travel freely through the universe when the universe was about 380,000 years old?
A) Magnetic fields had weakened to the point that they no longer interfered with light.
B) The universe had expanded and cooled enough for electrons to combine with nuclei to form neutral atoms.
C) Atomic nuclei were finally able to escape the plasma of the early universe.
D) Atoms started to combine to make the first molecules.
E) Fusion of hydrogen to make helium ceased.
B) The universe had expanded and cooled enough for electrons to combine with nuclei to form neutral atoms.
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An electron is moving at 
m/s perpendicular to a uniform magnetic field. If the radius of the motion is 18 mm, what is the magnitude of the magnetic field?
A. 70 mT
B. 0.95 mT
C. 9.3 mT
D. 1.6 
T
E. 8.9 nT
A wave whose wavelength is 0.500 m is traveling down a 500-m long wire whose total mass is 25 kg and is under a tension of 2000 N. (a) What is the speed of the wave on the wire? (b) Find the frequency of this wave
What will be an ideal response?
An asteroid revolves around the Sun with a perihelion 0.5 AU and an aphelion of 7.5 AU. What is its period of revolution?
a. 4 years b. 8 years c. 16 years d. 32 years e. 64 years
If a gasoline engine produces a hotter explosion, then the efficiency of the engine should
A. be the same B. go up C. go down