Q: Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
The River Lagan (from Irish Abhainn an Lagain, meaning ''river of the low-lying district''; Ulster Scots: Lagan Watter) is a major river in Northern Ireland which runs 53.5 miles (86 km) from the Slieve Croob mountain in County Down to Belfast where it enters Belfast Lough, an inlet of the Irish Sea.
A:
Which river system contains River Lagan?