Which of the following best explains why nuclear fusion requires bringing nuclei extremely close together?
A) Nuclei are attracted to each other by the electromagnetic force, but this force is only strong enough to make nuclei stick when they are very close together.
B) Fusion can proceed only by the proton-proton chain, and therefore requires that protons come close enough together to be linked up into a chain.
C) Nuclei normally repel because they are all positively charged and can be made to stick only when brought close enough for the strong force to take hold.
D) Nuclei have to be very hot in order to fuse, and the only way to get them hot is to bring them close together.
Question 2
What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
A) observations of the speeds of individual galaxies in clusters
B) measurements of how galaxy speeds away from the Milky Way have increased during the past century
C) observations of white dwarf supernovae
D) measurements of the rotation curve for the universe
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