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 long was the tributary of the Amazon that Crevaux descended to? Passage:The interior of French Guiana was virtually unknown and Crevaux decided to lead an exploration into its depths. On July 8, 1877 Crevaux traveled up the Maroni River where he encountered the Galibi and the Bonis. He left the Maroni to follow a tributary, the Itany River, along the way he visited the Roucouyenne and then followed an Emerillon trail over the spine of the Tumuk Humak Mountains. He descended the other side of the mountains to the Jari River, a tributary of the Amazon. By December 1877 he had reached the Brazilian city of Belem. He was nearly naked and had lost or used most of his possessions, and was believed by the Brazilian inhabitants to be an escaped French prisoner and was refused any help. He was eventually aided by a fellow Frenchman who bought him passage on a ship back to France. Upon returning to France, Crevaux gave an account of his journey to the Societe de Geographie and was made a "Knight" of the Legion d'honneur.
Links: a. Jari River b. Belem c. Aluku d. Emerillon
Output:
a