Concept:The mass method involves using mass media to reach many people at once, so its main drawback is the high cost involved.Explanation:Option A is correct because operating a mass method campaign is very expensive.Option B is wrong because the mass method saves time, so it is an advantage.Option C is wrong because reaching a larger audience is a benefit of the mass method.Option D is wrong because the mass method does not require the physical presence of an agent, which is also an advantage.Answer:A. It is very expensive to operate