---How does your father go to work? ---He often goes to work____ his car every day.
After half a day’s hard work, we made our classroom look as _______ as new.
_______ provides many hours of air for astronauts in space.
We are wondering ________ we can prevent people from polluting the earth.
The gravity on Mars is only about ________ of that on Earth.
It is hard to imagine what life __________ in 100 years.