谁帮我分析一下这个句子？
When I try to understand -------- that prevent so many Americans from being as happy asone might expect ， it seems to me that there are 2 causes。 A .why it does B. what it does C. what it is D. why it is 为什么用（What it is ）？要分析。谢谢。
When I try to understand what it is that prevent so many Americans from being as happy as one might expect, it seems to me that there are 2 causes。
这是一个when引导的状语从句,里面套了一个宾语从句,宾语从句里又套了一个定语从句。 拆分成三个句子: When I try to understand what。这是主句。 it is what。那是什么 (what 是understand的宾语,同时又引导了这个宾语从句) What prevent so many Americans from being as happy as one might expect (那东西)阻止这么多的美国人不能尽可能地高兴(感到幸福)。
整句话的翻译:当我努力地寻找是什么导致了这么多的美国人不能像他们所期待的那样幸福时,我发现好像有两种原因。
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