Up to now, no one has come up with a(n) explanation of why dinosaurs died out, and it remains a mystery.
The population of our country at present is ______ that 30 years ago.
Turn on the TV or open a magazine ______ you will often see advertisements showing happy, balanced families.
—Do you mind my smoking here? —______. This is a non-smoking room here.
At the class meeting, he made two suggestions, ______ were of great value.
His parents died when he was very young, so he was by his grandmother.