How does a rocket launch upwards? Choose the statement that best describes why a rocket goes up.
A) By expelling gas downwards, the rocket is propelled upwards.
B) By expelling gas, the rocket loses mass; because it is lighter, it rises up.
C) By expelling gas downwards, which pushes against the ground, the rocket is propelled upwards.
Question 2
In order for the cosmic microwave background to show hydrogen absorption lines at the redshift corresponding to the universe's age of 380,000 years, what condition would have been necessary?
A) There would have needed to be even warmer hydrogen clouds (which would contradict the observation of a uniform temperature in all directions).
B) There would have needed to be cooler hydrogen clouds (which would contradict the observation of a uniform temperature in all directions).
C) There would have needed to be ionized hydrogen (which would contradict the idea that the CMB originates at the time the universe cooled enough for protons and electrons to combine, making neutral atoms).
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