OG12 SC3 关于it的指代疑问！求助！

毛毛的臭脚丫

Although various eighteenth-and nineteenth-century American poets had professed an interest in Native American poetry and had pretended to imitate Native American forms in their own works, until almost 1900, schoolars and critics did not begin seriously to study traditional Native American poetry in native languages.
A until almost 1900, scholars and critics did not begin seriously to study  
B  until almost 1900 scholars and critics had not begun seriously studying
C  not until almost 1900 were scholars and critics to begin seriously to study
D  it was not almost until 1900 when scholars and critics began to seriously study
E  it was not until almost 1900 that scholars and critics seriously began studying

答案是E，但是想请教一下其中的it指代的是什么？为什么不会有指代不清的嫌疑？

jefferson911

it is/was...that/who...只是强调结构的标志吧，it应该是没有指代的。

lala974

up up

laria_line

指代that后面的scholars and critics seriously began studying这件事。