I can’t live on . . . . . . . . 500 dollars a month. A. a B. an C. the D. no article

In the sentence “I can’t live on 500 dollars a month,” the word “dollars” is used as a measure of quantity, not as a specific number of dollars. Therefore, it is not preceded by an article.

If the sentence were to be changed to “I can’t live on a 500 dollar bill,” then the word “dollar” would be preceded by the article “a” because it is referring to a specific dollar bill.