GWD-13-Q1

Althoughshe had been known as an effective legislator first in the Texas Senate and laterin the United StatesHouse of Representatives, not until Barbara Jordan’s participation in thehearings on the impeachment of President Richard Nixon in 1974 was she made anationally recognized figure, as it was
televised nationwide.A.later in the United States House of Representatives, not until Barbara Jordan’sparticipation in the hearings on the impeachment of President Richard Nixon in1974 was she made a nationally recognized figure, as it was
B.later in the United States House of Representatives, Barbara Jordan did notbecome a nationally recognized figure until 1974, when she participated in thehearings on the impeachment of President Richard Nixon, which were
C.later in the United States
House of Representatives, it was not until 1974 that Barbara Jordan became anationally recognized figure, with her participation in the hearings on theimpeachment of President Richard Nixon, which wasD.then also later in the United States House of Representatives, not until 1974did Barbara Jordan become a nationally recognized figure, as she participatedin the hearings on the impeachment of President Richard Nixon, being
E.then also later in the United States House of Representatives, Barbara Jordandid not become a nationally recognized figure until 1974, when she participatedin the hearings on the impeachment of President Richard Nixon, which was

这题B确实比较好。不过我有个小疑问：
从答案看来，在这个句子里，which定语从句，指代的前面的hearings，因为which后用了were。
那是不是也有指代不清的嫌疑，就是指代了inpeachment, 或者president Richard Nixon。
想知道这种情况下，which究竟优先指代谁呢？