The slime layer prevents deposition of calcium carbonate. When the slime sloughs away, it carries away which substance and leaves the pipe surface bare?

• A. zinc orthophosphate, ferrous hydroxide
• B. calcium carbonate, calcium carbonate
• C. calcium carbonate, the pipe surface becomes bare
• D. calcium carbonate, calcium oxide

Answer: (C)

The key idea is that a slime layer (biofilm) on pipe walls can prevent calcium carbonate from depositing as scale. When this slime layer sloughs off, it takes with it the calcium carbonate that had formed or begun to form, leaving the pipe surface bare and exposed.

So the substance carried away is calcium carbonate, and the pipe surface becomes bare. The other options don’t fit because they involve substances not typically associated with the slime’s detachment in this context, or they imply calcium carbonate remains on the surface, which isn’t consistent with the idea that the slime’s removal exposes a bare surface.