1.
You use ~ to describe the use of natural resources when this use is kept at a steady level that is not likely to damage the environment.
Try to buy wood that you know has come from a ~ source.
ADJ
2.
A ~ plan, method, or system is designed to continue at the same rate or level of activity without any problems.
The creation of an efficient and ~ transport system is critical to the long-term future of London.
ADJ