Which of these sentences is an example of a compound sentence?

a.
The athletes trained for months before they competed in the event last week.
c.
Seventh and eighth graders are preparing for their final exams.

b.
Kelvin spent the entire summer working at summer camp.
d.
The chicken satay was very flavorful, but the curry was so fragrant and warming.

Please select the best answer from the choices provided

A
B
C
D