Which words in the sentence are the adverb clause?
before they ate a snack, the children washed their hands in the sink.
a. the children washed their hands
b. in the sink
c. before they ate a snack

'Before they ate a snack' is the correct answer, it is the adverbial clause of time. A cannot be correct because that is an independent clause, and adverb clause has to be dependent. B isn't a clause at all.