Since the desert air was so dry, the bread quickly turned stale. in this sentence, which word serves as a clue that there is cause and effect . so, since, stale or quickly

The question is giving us the following sentence: "Since the desert air was so dry, the bread quickly turned stale" And the question is: in this sentence, which word serves as a clue that there is cause and effect?. The cause and effect in those sentences are the fact that the first phrase causes the second and they're connected by "since" - and this is the correct answer. Since means also "because".