为什么总是没有人理我呢 这一题没有人讨论过的

Q31:
                

Scholars who once thought Native American literatures were solely oral narratives recorded by missionaries or anthropologists now understand this body of work to consist of both oral literatures and the written works of Native American authors, who have been publishing since 1772.

1. Scholars who once thought Native American literatures were solely oral narratives
2. Scholars thinking of Native American literatures once solely as oral narratives, and
3. Scholars who once had thought of Native American literatures solely as oral narratives and
4. Native American literatures, which some scholars once thought were solely oral narratives
5. Native American literatures, which some scholars once, thinking they were solely oral narratives

我选C ,答案是A

不明白thought 后面不应该有that 吗

我选A，

觉得，凡是看到选项里有had done这样的过去完成时的，都得小心，看看是否有时间标志，表明主句的动作发生在过去的过去。

C选项里看不到这样的时间标志词。