The end of this era ushered in a period of "<span>B. foreign wars," since the Pax Romana had been a period of relatively unprecedented peace and prosperity for Rome. </span>
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He is also known for his discovery of the photoelectric effect, for which he won the Nobel Prize for Physics in 1921. Einstein developed a theory of special and general relativity, which helped to complicate and expand upon theories that had been put forth by Isaac Newton over 200 years prior.
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