The discovery of these tombs is _______ for scholars’ studying the history of the Tang Dynasty.
Many of them turned a deaf ear to his advice, _______ they knew it to be valuable.
Helen ______ her keys in the office so she had to wait until her husband ______ home.
The teacher, with 6 girls and 8 boys of her class, ______ visiting a museum when the earthquake struck.
With a lot of difficult problems ______, the newly-elected president is having a hard time.
—I must apologize for ______ ahead of time. —That’s all right.