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1. The reasons can be listed as follows. For one thing…For another…
2. What has contributed to this problem? In the first place…In the second place…
3. Perhaps the main/primary/most important reason is…
4. It is no easy to identify the reason for… which involve several complicated factors.
5. A number of factors could account for/lead to/contribute to/result in/be conductive to…
6. One may trace/attribute this problem to…
7. This problem may result from/ largely due to/stem from a combination of factors.
8. The explanation for this phenomenon involves many complex reasons…
9. There are a variety of reasons for this phenomenon.
10. There are three reasons for this.
