Which is a compound sentence?

 - In 1900, the U.S. state with the largest population was New York.
 - Rumors about Mayor Schmidt were spreading like wildfire, so she set the record straight at a press conference.
The first sentence is the compound sentence. It is made up of two independent clauses joined by the coordinating conjunction so.
Rumors about Mayor Schmidt were spreading like wildfire, so she set the record straight at a press conference.