Which of the following sentences has a comma splice?
a. all of these

b. the strong smell of garbage was hard to ignore, it seemed pretty clear that nothing had gotten done while she was gone.

c. there was a bit of confusion as the race began, but it wasn't long before everyone figured it out and the runners were on their way.

d. as they walked down the road, all he could think about was how happy he was in that moment.