Question

Before the bell rang, all the students were required to neaten up the areas around their desks.

Answer 1

C - before the bell rang

Answer 2

An adverb clause contains a subject and a verb. It is usually placed next to a subordinating conjunction, which prevents it becoming a full thought. The clause itself should be able to answer one of the Why? When? Where? How? 

The answer to your question is C) Before the bell rang because it meets all of the requirements, including the When? question.

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