Answer: A) where oases are plentiful but distant from each other.
An adverb clause is a group of words that function as an adverb in a sentence. This means that it can modify or describe verbs, adverbs and adjectives. In this case, the adverb clause elaborates on the "where" of the action. However, adverb clauses can also elaborate on "when," "why," "how" and "how much."