A. The user's browser renders the html code as a visual web page.
The correct answer is A. This is the case of using comma before a coordinating conjunction <em>and</em> because it links two independent clauses. We can observe that there two parts of this full sentence and each part is independent from each other. Both of them have their own subject and verb. Therefore, we are using a comma before <em>and</em>.
C. Not only the muffler but also the spark plugs need to be replaced. ☺
