1. There are probably three/many/several/varieties of reasons/causes for this dramatic/significant increase/decline in… 引起……显著增长(下降)的原因有三个(许多、几个、很多)。
2. Some reasons can explain this trend. First of all,… Secondly,… Furthermore,… 一些原因可以解释这一趋势。首先,……;其次,……;另外,……
3. Why…? For one thing,… for another,… 为什么……?第一,……;第二,……。
4. The causes for…are varied. They include…Perhaps the main cause is… 造成……的原因有很多,包括……。主要的原因可能是……
5. Perhaps the main/primary/fundamental/most important reason is… 也许,最主要(根本、重要)的原因是……
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6. The reason for this is not far to seek. The first reason is that…The second one is…The third is… For all this, the main cause of…is due to… 这一问题的原因不难发现。第一个原因是……。第二个是……。第三个是……。综上所述,……最主要的原因应该是……
7. It is no easy task to identify the reasons for this phenomenon which involves several complicated factors.要找出这一现象的原因并非易事,因为它涉及若干复杂的因素。
8. Certainly,…is not the sole reason for…;…is also responsible for the change/problem.当然,……并不是唯一的原因。还有……也导致了这一变化(问题)的产生。
9. You dont have to look very far to find out/identify the reason why… 你不必花费很大的力气就会找到……的原因。
10. There are numerous reasons why…,and I would explore only a few of the most important ones here.……的原因有很多,在此我只想探讨其中几个最重要的原因。