．It’s best to plant trees(植树)in _____．

 Some accidents have happened on the road recently. So we should pay attention to the __________ problems.

Of the seven days in a week, Saturday is said to be the most popular _______ for a wedding in some countries.

Mr. Wu is a teacher with _______. He often tells us some of his unusual _______ in America.

If H2 burn in O2, we can get _____.