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 many years has Serco been operating the NorthLink Ferries? Passage:Serco operates the Santander Cycles scheme (for Transport for London) and the NorthLink Ferries Northern Isles lifeline ferry service in Scotland. Serco also has a joint venture with Abellio, the international arm of Dutch national rail operator Nederlandse Spoorwegen, to operate passenger trains in the UK. Serco-Abellio has a 25-year concession for Merseyrail in Liverpool until 2028 and operated the Northern Rail franchise from December 2004 until March 2016. Serco's Home Affairs division, run by Tom Riall, also operates speed camera systems throughout the UK and, until November 2013, designed, wrote and tested the software that controls the matrix message signs, signals, emergency roadside telephones (SOS) and traffic monitoring on England's motorway network including, until 2011, the National Traffic Control Centre. It previously operated the Docklands Light Railway.
Links: a. Northern Rail (Serco-Abellio) b. NorthLink Ferries c. Docklands Light Railway d. Liverpool
Output:
b