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.

Example Input: Question: How many people died in the incident that contaminated about 30,000 km?  Passage:Natural strontium is a mixture of four stable isotopes: Sr, Sr, Sr, and Sr. Their abundance increases with increasing mass number and the heaviest, Sr, makes up about 82.6% of all natural strontium, though the abundance varies due to the production of radiogenic Sr as the daughter of long-lived beta-decaying Rb. Of the unstable isotopes, the primary decay mode of the isotopes lighter than Sr is electron capture or positron emission to isotopes of rubidium, and that of the isotopes heavier than Sr is electron emission to isotopes of yttrium. Of special note are Sr and Sr. The former has a half-life of 50.6 days and is used to treat bone cancer due to strontium's chemical similarity and hence ability to replace calcium. While Sr (half-life 28.90 years) has been used similarly, it is also an isotope of concern in fallout from nuclear weapons and nuclear accidents due to its production as a fission product. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia. The 1986 Chernobyl nuclear accident contaminated about 30,000 km with greater than 10 kBq/m with Sr, which accounts for 5% of the core inventory of Sr.
 Links: a. Rubidium b. Mass number c. Electron capture d. Chernobyl disaster
Example Output: d

Example Input: Question: In what year was CIA pilot Francis Gary Powers born? Passage:Strategic Air Command was formed by the United States Air Force after World War II to provide an active defense against any surprise attack by the Soviet Union. Though it had been an ally against Germany and Japan during World War II, by 1948 the Soviet Union showed a propensity to instigate problems with Britain, France and the United States. In August 1949, the Soviet Union detonated its first nuclear weapon and by the early 1950s had detonated a hydrogen bomb. The war of words between the two superpowers escalated during 1950s and 1960s into a nuclear arms race. By 1970 the United States was using a "Triad Defense System" composed of nuclear submarines armed with nuclear missiles, land based intercontinental ballistic missiles with nuclear warheads and bombers capable of delivering hydrogen bombs on enemy targets. The Boeing B-52 Stratofortress bomber had been designed in the early 1950s by Boeing Aircraft Company to give the United States Air Force the capability of delivering nuclear weapons far inside the territory of Soviet Russia. The planes were to fly at high altitude with enough fuel to hit their target. In May 1960, the Soviet Union made known its capability to shoot such high altitude planes out of the sky by using a surface to air missile to strike CIA pilot Francis Gary Powers' U-2 spy plane over Russian territory. From that point on, the high-altitude B-52 had to be modified to conduct missions at low level, something it was not intended to be.
 Links: a. Strategic Air Command b. Intercontinental ballistic missile c. Intercontinental ballistic missile d. Francis Gary Powers
Example Output: d

Example Input: Question: When was the football club Stanislas joined at age 10 founded? Passage:Born in Kidbrooke, London, Stanislas first joined West Ham United as a schoolboy at the age of 10. In May 2006, Stanislas signed a three-year academy scholarship. He made his Premier Academy League debut in a 3–2 defeat to Watford on 8 April 2006, at the age of 16. He became a regular for the Academy in the 2006–07 season, appearing 26 times and scoring 9 goals as West Ham finished as runners-up to Arsenal. He carried his goalscoring form into the 2007–08 season, scoring 10 goals in 24 appearances. He made his Premier Reserve League debut on 29 August 2007, in a 2–1 defeat to Aston Villa. His first goals came in the 8–0 rout over Derby County, as he netted a brace. He was part of the first-team squad that travelled to North America in the summer of 2008 for the pre-season tour. He started the 2008–09 season in the reserves, scoring his first goal of the season on 21 October 2008, in a 1–0 home win over Arsenal.
 Links: a. Arsenal F.C. b. Watford F.C. c. 2007–08 Premier Academy League d. West Ham United F.C.
Example Output:
d