A. If you guessed (b), you've nailed our pop quiz.
The photo above was taken at low tide, when water from the Adriatic Sea and the Venetian Lagoon had receded enough to show moss growing on the brick walls of a canal.
The picture below is similar but more worrisome, because the moss shows that acqua alta, a.k.a. "high water," has soaked into the brick above a building's marble foundations. After a few decades of repeated exposure to salt water, the brick will start to crumble, and chunks of the damaged building wall (or canal walls, for that matter) are likely to begin falling into the canal.
Venice has an ongoing program of canal maintenance, and in recent years the city has responded to ever-higher acqua alta levels by replacing damaged water-damaged brick with layers of white marble or Istrian stone. The stone is far more resistant to salt water than brick is, and it's said to offer protection against water damage for hundreds of years.