Which sentence is the correctly written compound sentence?
a. once each year, chincoteague firefighters round up the ponies, set up an auction, and sell some.
b. once each year, chincoteague firefighters round up and sell some ponies at auction.
c. once each year, chincoteague firefighters round up the ponies and sell some at auction.
d. once each year, chincoteague firefighters round up the ponies, and some are sold at auction.