I think humour is a vital form of entertainment, ____ it makes us laugh and laughing keeps us healthy.
When we got to the airport, the plane was starting _____.
The river _____ are covered with trees is very long.
-----Tim, please don’t talk in class. -----Pardon? -----I _____ talk in class.
When Jack arrived, he learned Mary _____ for almost an hour.
The speaker mentioned some writers and books _____ were known to us.