Given the task definition and input, reply with output. 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 Ice Mountain is alongside? Passage:Ice Mountain is an arc-shaped forested ridge of the Allegheny Mountains, part of the Ridge-and-valley Appalachians. It is above mean sea level at its summit. Ice Mountain is a large mass of Devonian Oriskany (Ridgeley) sandstone and Marcellus shale with numerous bare rock slopes and vertical cliffs. Ice Mountain lies on the west side of the Timber Mountain anticline and to the west of North Mountain fault, which places it on the Martinsburg allochthonous sheet. Ice Mountain is situated along North River and is known for the several hundred yards of ice that form at its base all year long. At its southern end overlooking the community of North River Mills is located Raven Rocks, a set of stone chimney outcrops. Raven Rocks is in height above mean sea level with vertical cliffs measuring nearly in height. Raven Rocks were named because of the presence of ravens during pioneer days. The present Raven Rocks is the remaining vestige of a once towering cliff that overlooked the North River. Geologically, Ice Mountain is a northern extension of North River Mountain.
 Links: a. Raven b. North River (Cacapon River) c. Sandstone d. Allochthon

b