Which is a compound sentence?

 - Helen paints in her studio at night, and she paints outside under the oak tree during the day.
 - Before the race, the athletes enjoyed a healthy breakfast of oatmeal, fruit, and tea.
The first sentence is the compound sentence. It is made up of two independent clauses joined by the coordinating conjunction and.
Helen paints in her studio at night, and she paints outside under the oak tree during the day.