Here's why:
* It contains two clauses:
* Independent clause: "The audience applauded after the first act." (This clause could stand alone as a complete sentence.)
* Dependent clause: "Although the actors were very nervous" (This clause cannot stand alone as a complete sentence. It depends on the independent clause for its meaning.)
* The dependent clause begins with a subordinating conjunction: "Although".
Therefore, the combination of the independent clause and the dependent clause connected by a subordinating conjunction makes it a complex sentence.