26 下圖是患者矯正後 6 個月的情形,此時最好選用那一種維持器(retainer)以改善此復發情形? (A)活動式維持器(removable retainer) (B)固定式維持器(fixed retainer) (C)主動式維持器(active retainer) (D)功能性矯正裝置(functional appliance)