7.
which sentence best shows correct comma usage?
corn, cotton, fruit, wheat beans, coffee tomatoes, and rice are grown in mexico.
corn, cotton, fruit, wheat, beans, coffee, tomatoes and rice are grown in mexico.
corn, cotton, fruit, wheat, beans, coffee, tomatoes, and rice are grown in mexico.
corn, cotton fruit wheat beans coffee tomatoes, and rice are grown in mexico.

The sentence that best shows correct comma usage is the following one: Corn, cotton, fruit, wheat, beans, coffee, tomatoes, and rice are grown in Mexico.

Explanation:

Commas are used to separate three or more words, phrases, or clauses in a series. In other words, when three or more items are being listed in a sentence, a comma goes between each item in the list (corn, cotton, fruit, wheat, beans coffee, tomatoes). What is more, the conjunction and is placed between the last two items in the series. The comma before the conjunction is called Oxford comma. The Oxford comma is the final comma in a series of things.