The correct answer is A) maintaining a normal level of scavenging agents. Scavenging agents, such as sulfites or hydrazine, are added to boiler water to remove dissolved oxygen. This is important because dissolved oxygen can cause corrosion in the boiler system. By maintaining a normal level of these scavenging agents, you can be fairly certain that the dissolved oxygen is being properly removed, even if accurate onboard testing is difficult. The other options are incorrect because: B) maintaining low boiler water pH does not directly address dissolved oxygen removal, C) testing for total dissolved solids does not specifically target dissolved oxygen, and D) surface blows may help remove some dissolved oxygen, but do not address the root cause like using scavenging agents.