The main complaint was that the Catholic Church sold indulgences. The extremely corrupt Catholic Church claimed that buying them would give you or a relative a free ticket out of Purgatory/Hell or at least lessen their sentence. Many people of the time disagreed with this because they believed that providence or actions alone determined your fate.
It is literally (c), that question was easy
America got involved in WWI because Germany attacked a US ship that was going to Britain.
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The roman commoners were called plebeians