--- I think the waiter has forgotten us. We ________ here for over half an hour! --- I think you’re right. He probably thinks we have already ordered.
Exploring _____space is exciting, yet dangerous. There is _____need for both robots and humans for space exploration.
Life is like time. Once _____, it never returns.
The company gave away 300 dictionaries to the students at the beginning of the term, and _____500 the next term.
While many believe Benjamin Franklin to have discovered electricity, recent findings seem to show _____.
— Can I smoke in this hall? — No, you can’t. If you _____, please go to the smoking room.