While circulating heated sulfuric acid through a feed water heater of a flash evaporator for the purposes of scale removal, what would be the indication of successful removal of the scale?

Correct Answer: Option A — No change in color (remaining at red with a pH of 2.0) and no more venting of carbon dioxide gas.

Successful descaling is indicated when the acid solution remains at its initial low pH (red, pH 2.0), showing it has not been neutralized by scale. The cessation of carbon dioxide gas venting confirms that the chemical reaction between the acid and the scale has concluded, signifying complete removal.