The answer is: A) if you don’t buy them in advance.
An adverb clause functions as an adverb, modifying an adjective, verb or another adverb. It contains a subject and a verb, and it provides information as regards how, when, where and why the action happens.
Adverb clauses are also dependent clauses because they cannot stand on its own - they do not express complete thoughts by themselves. As a result, they need to join a main clause in order to make sense. They are joined to main clauses through subordinating conjunctions, such as while, although and if.
