We were swimming in the lake _______ suddenly the storm started.
| A．until | B．while | C．when | D．before |
The environment in our city ____ greatly if we make every effort in the near future.
| A．improved | B．was improved |
| C．has improved | D．will be improved |
________ the girls in our class ________ over twenty.
| A．A number of, are | B．The number of, are |
| C．A number of, is | D．The number of, is |
We planted _______ trees last year.
| A．hundreds of | B．hundred of | C．five hundred of | D．five hundreds of |
The ________ of the island was quite an event in the world.
| A．discover | B．discovery | C．discovered | D．discoveries |
My smother ________ the e-mail invitation, but she realized she had forgotten to invite John.
| A．sent out | B．sent up | C．sent to | D．send out |