Which is a compound sentence?

 - Rumors about Mayor Wagner were spreading like wildfire, so she set the record straight at a press conference.
 - At the auction, several paintings by famous artists were for sale, including one by Pablo Picasso.
The second sentence is the compound sentence. It is made up of two independent clauses joined by the coordinating conjunction so.
Rumors about Mayor Wagner were spreading like wildfire, so she set the record straight at a press conference.