If we _______ now to protect the environment, we'll live to regret it.
| A．hadn't acted | B．haven't acted |
| C．don't act | D．won't act |
Bob called to tell his mother that he couldn’t enter the house, for he ________ his key at school.
| A．had left | B．would leave |
| C．was leaving | D．has left |
It was only after he had read the papers _____Mr. Gross realized the task before him was extremely difficult to complete.
| A．when | B．that | C．which | D．what |
When I arrived, Bryan took me to see the house，_____ I would be staying.
| A．what | B．when | C．where | D．Which |
_______ more and more people speak the global languages of English,
Chinese, Spanish, and Arabic, other languages are rapidly disappearing.
| A．When | B．While |
| C．As | D．As soon as |
A small plane crashed into a hillside five miles east of the city, _____all four people on board.
| A．killed | B．killing |
| C．kills | D．to kill |