The below sentence was taken from one of TOEIC questions. I chose "that"(relative pronoun), but the answer was "which". I understand if there is a comma after the word 'month', then it should be "which". There is no comma in this case.
Why does this have to be "which" in the sentence?
TOEIC question:
Ms. Lam is stepping down as CFO next month ------- means the company must seek a replacement immediately.
Thanks.
In your sentence, which is used because it introduces a non-defining relative clause. Such clauses provide additional information about a noun, but they can be removed from the sentence without changing the essential meaning of the main clause.
In the sentence you provide, Ms. Lam is stepping down as CFO next Monty is the main clause, and which means the company must seek a replacement immediately is an additional piece of information about that fact. As non-defining clauses are usually set off by commas, which is the correct relative pronoun to use here, even though there is no comma in the sentence.
If you had used that instead of which, it would imply a defining relative clause, which would change the meaning of the sentence, suggesting that there's only one CFO stepping down next month, and that specific CFO requires an immediate replacement.
As @BillJ has commented, and @Enguroo explained in the other answer, which is needed.
The entire clause Ms. Lam is stepping down as CFO next month is referred to. Hence the relative clause needs to be a non-defining one, that is, which is needed.
Cambridge Dictionary explains the use of relative clauses to refer to a whole clause:
Some relative clauses refer to a whole clause ... We always use which to introduce these clauses [emphasis added].
As @BillJ said, a comma is not a foolproof test.
The absence of commas does not imply only that can be used. This is because the relative pronoun which can be used in both defining and non-defining clauses. In our case here, it is the latter, and I usually set off such a clause with commas.
