Walking is a nice way to relax. ______, it can help reduce body weight if you do it regularly.
He stumbled(绊倒)on the _______ ground and fell flat on his face.
Many students today suffer from ________ problems, since they are under too much pressure.
________ energy is generated from natural resources—such as sunlight, wind, rain, tides and so on.
Effective measures have been taken to prevent _______pollution.
It worried her a bit ____ her hair was turning grey.