Which sentence uses commas correctly?
a.
We toured, an organic farm, a conventional farm, a food processing plant, and a grocery store.
b.
We toured an organic, farm, a conventional, farm, a food processing plant, and a grocery store.
c.
We toured, an organic farm a conventional farm, a food processing plant and a grocery store.
d.
We toured an organic farm, a conventional farm, a food processing plant, and a grocery store.