Concept:The error is in part b) due to incorrect use of passive voice where active voice is needed.Explanation:The phrase "was stopped aching" uses the passive form ("was + V3") incorrectly.The sentence describes a natural action of the tooth, not an action done to it.Active voice should be used: "my tooth stopped aching".The correct structure is subject + verb (active), not subject + "was" + past participle.Thus, the error is in part b).Answer:B