Which statement about the cosmic microwave background is not true?
A) It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.
B) Its spectrum has the shape of an essentially perfect thermal radiation spectrum.
C) With the exception of very small variations, it appears essentially the same in all directions in which we look into space.
D) It is thought to be radiation that began its journey to our telescopes when the universe was about 380,000 years old.
A) It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.
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Dimensional Analysis: When dividing several quantities
A. the quantities can have any combination of units. B. the quantities must all have exactly the same units. C. the quantities must all be dimensionless.
A 120 N picture hangs from a nail in the wall. The maximum shear stress the nail can sustain is 1.00 
The smallest diameter the nail can be is
A. 10.5 mm. B. 12.4 mm. C. 13.7 mm. D. 14.4 mm. E. 15.9 mm.
The atmosphere of Titan is composed mostly of
A) oxygen. B) methane. C) carbon dioxide. D) hydrogen. E) nitrogen.
If we imagine that 1 in 10 million stars would eventually give rise to a civilization on an orbiting planet and assuming that there are 100 billion stars in our Milky Way galaxy, how often should a civilization randomly appear over a 5 billion year
period? A) once every 50 million years B) once every 50,000 years C) once every 500,000 years D) once every 5 million years