Which of these is an example of a compound sentence?

a.
The artist uses oil paints now, but he used to use watercolors exclusively early in his career.
c.
Bodybuilding can cause serious injury if improper form is used when lifting weights.
b.
The dancers looked so graceful on the dance floor.
d.
none of these

Please select the best answer from the choices provided

A
B
C
D