Answer: The adverb clause in the sentence provided is "as though nothing had happened". The subordinating conjunction is "as though" and the word modified is "greeted".
Explanation: An adverb clause is a subordinate clause, that is to say a clause that cannot stand alone as a sentence and that is introduced by a subordinating conjunction. In the sentence provided, "as though nothing had happened" does not express a full idea by itself and it is acting as an adverb; therefore, it is an adverb clause. It has been introduced by the subordinating conjunction "as though" and it is modifying the main verb of the independent clause ("the girls greeted each other"): "greeted".