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...
6)We have good reason to believe that...
例：There are three reasons for the changes that have taken place in our life. Firstly, people’s 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.
注：如考生写第一个句子没有把握，可将其改写成两个句子。如：Great changes have taken place in our life. There are three reasons for this. 这样写可以避免套用中的表达失误。