You will be given a definition of a task first, then some input of the task.
In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.

Question: How many years before the Fifteenth Amendment was the First Amendment ratified? Passage:In the United States, the Fifteenth Amendment to the United States Constitution, ratified in 1870 during the Reconstruction era, provided that "The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude." This amendment aimed to guarantee the right to vote to African Americans, many of whom had been enslaved in the South prior to the end (1865) of the American Civil War and the 1864-1865 abolition of slavery. Despite the amendment, however, blacks were disfranchised in the former Confederate states after 1877; Southern officials ignored the amendment and blocked black citizens from voting through a variety of devices, including poll taxes, literacy tests, and grandfather clauses; violence and terrorism were used to intimidate some would-be voters. Southern blacks did not effectively receive the right to vote until the Voting Rights Act of 1965.
 Links: a. Fifteenth Amendment to the United States Constitution b. Poll taxes in the United States c. Slavery in the United States d. Fifteenth Amendment to the United States Constitution
Output:
a