—Excuse me, madam. Could you please tell me _________ the bus arrives?—Sure. At 9:00.
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.
_______you come or not is up to you.