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 is the river that bordered the land occupied by Bavarians? Passage:In the meantime, the Germanic tribe of the Bavarii (Bavarians), vassals of the Franks, had developed in the 5th and 6th century in the west of the country and in what is now known as Bavaria, while what is today Vorarlberg had been settled by the Alemans. In the northern alps the Bavarians had become established as a stem dukedom around 550 AD, under the rule of the Agilolfings until 788 as an eastern outpost of the Frankish Empire. At that time the lands occupied by the Bavarians extended south to current South Tyrol, and east to the river Enns. The administrative center was at Regensburg. Those groups mixed with the Rhaeto-Romanic population and pushed it up into the mountains along the Puster Valley.
Links: a. Enns (river) b. Francia c. Enns (river) d. South Tyrol
Output:
a