他对我们的安全负有责任。He is________ ________our safety.
You will not succeed (除非) you work hard.
The British teacher is very humorous. He always tells us (笑话 ).
Hainan is in the (南方)of China.
In autumn, quite a few (树叶) turn yellow and fall off the trees.
Computers were (发明) not long ago, but they are very useful for us.