Answer:
D. Since
Explanation:
Cause and effect refers to an event which causes another event to occur. In that respect, the conjunction since means because or for the reason that. As a result, the expression since the desert air was so dry implies that the aridity of the air in the desert caused the bread to become dry, hard or no longer fresh.
The rest of the alternatives are incorrect because so means very or to a great degree, stale is an adjective that means hard, rancid and unpleasant to eat, and the adverb quickly means fast.
