1)There are three reasons for this.
2)The reasons for this are as follows.
3)The reason for this is obvious.
4)The reason for this is not far to seek.
5)The reason for this is that...
例： There are three reasons for the changes that have taken place in our
life.Firstly,peoples living standard has been greatly improved.Secondly,most
people are well paid,and they can afford what they need or like.Last but not
least,more and more people prefer to enjoy modern life.