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: When was the company that built NSB Class 73 established? Passage:NSB Class 73 () is a class of 22 electric multiple units built by Adtranz for the Norwegian State Railways. The four-car trains were modifications of Class 71, which was again based on the Swedish X2. The A-series consists of 16 intercity trains; they were delivered in 1999 and 2000 and are used on the Bergen, Dovre and Srland Lines. The intercity service was branded as Signatur until 2003. The B-series consists of six regional trains delivered in 2002 and used on the stfold Line. The regional trains were originally part of the Agenda concept. The trains have a power output of and a maximum speed of . They have an overall length of and have a capacity for 208 seated passengers in the A-series and 250 in the B-series. The trains have a tilting mechanism allowing for faster travel through curves.
Links: a. Adtranz b. Bergen Line c. stfold Line d. Electric multiple unit
a