[問題]朗文托福聖經 p226 Q10

[dsegrl]

The report on the nuclear power plant indicated that when the plant had gone on line ___ unsafe.
(A) and it had been
(B) it had been
(C) had been
(D) that it had been

答案是(B),但我覺得 "when the plant had gone on line "是名詞子句
當主詞用,那(B)不就多了"it",請各位高手解答一下,謝謝.

[StanleyHuang]

我覺得 when the plant had gone on line 是副詞子句放句首，主詞應該是 the plant
原句就會是 the plant had been unsafe when it had gone on line.