Which is a compound sentence?

 - Our cat has been scratching himself more than usual, so I checked him for fleas.
 - The famous Venus de Milo statue was found on Milos, a volcanic Greek island in the Aegean Sea.
The first sentence is the compound sentence. It is made up of two independent clauses joined by the coordinating conjunction so.
Our cat has been scratching himself more than usual, so I checked him for fleas.