gwd-3-31

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

答案是a,不懂~~

B ，没有主语，

c ，once 和had thought 矛盾，加入and后，句子结构不成立

d，e, 主语错误，和now understand矛盾

同题，请教为什么C错？因为后面有“since 1772”前面的句子应该可以使用完成时态吗？

谢谢