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: What book first introduces the white wizard? Passage:Meanwhile, the wizard Gandalf the White and the hobbit Pippin arrive at Minas Tirith to warn Denethor, the Steward of the Throne, about the upcoming war—only to discover that the Steward has become insane and means to take his own life. Frodo and Samwise continue toward Mount Doom (eluding Ringwraiths and infiltrating a "battalion of orcs" in the process) only to be attacked by Gollum. As Sam holds Gollum off, Frodo reaches the Crack of Doom; but is subverted by the Ring. At the same time, Gondor's neighboring country, Rohan, helps it claim victory in the Battle of Pelennor Fields, where King Théoden and the Witch-King of Angmar are slain. Upon his own arrival, Aragorn plans to confront Sauron at the gates of Mordor. Here, he quarrels with the Mouth of Sauron and the two armies prepare for battle.
 Links: a. Aragorn b. Gandalf c. Gandalf d. Nazgûl
Output:
b