Which one of the following sentences containes an unnecessary comma.

A. We had to decide among steak, lobster, and pork roast.

B. Before we could enter the building, we had to show our identification badges.

C. The car that was used as a demonstration in the race, is owned by cambridge motor company.

D. The tree, which has been part of the community for more than 75 years, was struck by lightning.

http://grammar.ccc.commnet.edu/grammar/commas.htm

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