Simplify the following text for a Grade 7 native English-speaking person.
Original: The Kwun Tong Line was extended across the harbor on 5 August 1989 to Quarry Bay Station , which became an interchange station for the Kwun Tong Line and the Island Line .
Simplified: The Kwun Tong Line was extended across the harbor to Quarry Bay , which became an interchange station for the Kwun Tong Line and the Island Line . The extension was launched on 5 August 1989 .