Japan's coastline is rugged, because Japan is very mountainous
A coastline is the land that borders any body of water, but mostly oceans, seas, etc.
Coastlines are part of ocean that act as the borded line of that sea.
It's where land and sea meet. So there are beaches, cliffs and land sloping into the sea.
I think WE control coastlines, because we put rubbish in the sea, that could be recicled! Actually, the answer is both. The coastline controls us in the sense that it determines where we can build...