Which is a compound sentence?

 - In 1900, the U.S. state with the largest population was New York.
 - First, Dean planted the geraniums in a clay pot, and then he placed the pot on a sunny windowsill in his kitchen.
The second sentence is the compound sentence. It is made up of two independent clauses joined by the coordinating conjunction and.
First, Dean planted the geraniums in a clay pot, and then he placed the pot on a sunny windowsill in his kitchen.