Advantage vs. profit

[micha]

Synonym Search, Intermediate level

ESL/EFL Test #121 "Synonyms for profit and benefit", question 6

This shop sells jeans at 50 dollars there by making a ......... of about 30 dollars as the outlay is only 20 dollars a pair.

(a) advantage
(b) profit
(c) account
(d) capital

Synonym Search, Intermediate level

ESL/EFL Test #121 "Synonyms for profit and benefit", answer 6

This shop sells jeans at 50 dollars there by making a profit of about 30 dollars as the outlay is only 20 dollars a pair.

Correct answer: (b) profit

Your answer was: correct
_________________________

why the answer is b. why not the answer is a

[Pamela]

First if you want to use advantage you should use the article an but not a .