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 many years after the first Historic Mechanical Engineering Landmark status was granted did the Lookout Mountain Incline Railway receive their landmark status? Passage:The Lookout Mountain Incline Railway is a inclined plane funicular railway leading to the top of Lookout Mountain from the historic St. Elmo neighborhood of Chattanooga, Tennessee. Passengers are transported from St. Elmo's Station at the base, to Point Park at the mountain summit, which overlooks the city and the Tennessee River. It is just a short drive to three of Chattanooga's main tourist attractions, Ruby Falls, Cavern Castle, and Rock City. The railway is approximately in length (single-track except for a short two-track passing loop at the midway point, allowing operation of two cars at one time). It has a maximum grade of 72.7%, making it one of the world's steepest passenger railways. It obtained Historic Mechanical Engineering Landmark status in 1991. The cable system for the cars was made by the Otis Elevator Company.
Links: a. List of Historic Mechanical Engineering Landmarks b. Funicular c. Chattanooga, Tennessee d. Funicular
a