Friendship is both ________ source of pleasure and ________ component of good health.
If something ________, it becomes less and less and eventually disappears completely.
It’s the first time ______ he has helped us.
Here are two pairs of shoes, ______ you may choose.
He plays well or badly according to his mood (心情), so he’s a very _________ player.
No one knows exactly ______ this important discovery was made.