Definition: 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.
Input: Question: How many Presidents have died while still in office? Passage:McKellar twice served as President pro tempore of the United States Senate, commencing in 1945, the first to hold the position under the system that has prevailed since of reserving it for the most senior member of the majority party. When Harry Truman became president in April 1945, upon FDR's death, the vice presidency became vacant (the mechanism for filling intra-term vacancies had not yet been created by the 25th Amendment), and so McKellar became the permanent presiding officer of the United States Senate. Also, as the President pro tempore of the Senate had, prior to 1886, been second in the presidential line of succession, behind only the vice president, Truman viewed McKellar as the logical wartime replacement for himself, and asked McKellar to attend all Cabinet meetings. In 1947 Truman successfully lobbied Congress to pass a new Presidential Succession Act restoring both the Speaker of the House and the President pro tempore of the Senate to the succession ahead of the Cabinet secretaries. By the time the law came into effect, McKellar was no longer in the position of President pro tempore, because Republicans held the majority in the 80th Congress. Truman vetoed the Taft-Hartley Act in 1947 to restrict labor unions, which McKellar voted in favor of. As such Truman selected Alben Barkley of Kentucky as his running mate in the 1948 presidential election. When Democrats regained control as a result of the 1948 elections, McKellar again became President pro tempore. He was therefore second in line for the presidency (behind the Speaker of the House) from January 3, 1949 until January 20, 1949, when Alben Barkley took office as Vice President of the United States.
Links: a. President of the United States b. President of the United States c. Vice President of the United States d. 1948 United States Senate elections
Output:
a