Farmers here _________keeping the soil rich and _________diseases.
| A．focus on…free of | B．center … free from |
| C．attend on… free of | D．depend on… free from |
There is no doubt ____ John will come by bus.
| A．if | B．why | C．that | D．how |
Before you leave the lab, the doors and windows ____shut in case of rain.
| A．is | B．are | C．is to be | D．are to be |
Did John ____ the general cleaning yesterday afternoon?
| A．join | B．attend | C．take | D．take part in |
To enjoy the scenery, Irene would rather spend long hours on the train ___travel by air.
| A．as | B．to | C．than | D．while |
For some students, the computer serves only _____ a tool that is used to play games.
| A．of | B．in | C．as | D．to |