Although she had been known as an effective legislator first in the Texas Senate and later in the United States House of Representatives, not until Barbara Jordan’s participation in the hearings on the impeachment of President Richard Nixon in 1974 was she made a nationally recognized figure, as it was televised nationwide.

A.
later in the United States House of Representatives, not until Barbara Jordan’s participation in the hearings on the impeachment of President Richard Nixon in 1974 was she made a nationally recognized figure, as it was
B.
later in the United States House of Representatives, Barbara Jordan did not become a nationally recognized figure until 1974, when she participated in the hearings on the impeachment of President Richard Nixon, which were
C.
later in the Untied States House of Representatives, it was not until 1974 that Barbara Jordan became a nationally recognized figure, with her participation in the hearings on the impeachment of President Richard Nixon, which was
D.
then also later in the United States House of Representatives, not until 1974 did Barbara Jordan become a nationally recognized figure, as she participated in the hearings on the impeachment of President Richard Nixon, being
E.
then also later in the United States House of Representatives, Barbara Jordan did not become a nationally recognized figure until 1974, when she participated in
the hearings on the impeachment of President Richard Nixon, which was
答案是选B,  虽然我也选B,  但我觉得B 这个which是不是用的太不好了, 从意思上来讲他应该指代的是hearings 吧,  但我想为什么不能指代impeachment 呢,如果可以指代的话，  那B 也有指代不清的问题啊 ?  

impeachment是单数，后面是which were，所以which不可能指代impeachment，只能指代hearings。