As I've read an article from a popular blog, there were numerous answers by Mr. Weisman's book. As for what is the best answer for your question, it would be letter B. <span>Clearing out clogs in the sewer systems. It is what keeps the New York City intact as it is. If it weren't for the maintenance workers or even fixing steam pipes, the place would probably sunk-because of flood. </span>
When you meet someone, you should say hello, smile and shake hands. Parallel structure entitles that all the verbs be written in the same for. In the future, try reading the sentences out loud and the poorly written sentences will make themselves known. Hope this helped.
C. In my opinion is the most correct answer