听力原文： Two rail companies were fined a total of ￡325,000 yesterday for "an unsafe system of work practices" that led to the death of a 22-year-oid student.
Michael Mungovan, studying at Brunel University, was working a night shift with a track maintenance crew near Vauxhall station, south London, in October 2004, when he was struck from behind by an empty train and killed.
The court was told that Mr. Mungovan had been given only minimum training. He had worked a few shifts, but never on a live running line.
Why were two rail companies fined？

A. Because they asked young students to work at night.
Because their train was not safe.
C. Because they hired college students illegally.
D. Because their work practice system was unsafe.