— Kang Ming, we are going swimming in Jun Xi River this Saturday. Will you go with us?      

— Yes, I'd love to, but the water there is much _____ now. [ ]

A. cleaner

B. dirtier

C. warmer

患者男性，5岁6个月，因“右下后牙肿痛2天”来诊。口腔检查：右下5乳牙冠大部分破坏，龋洞较深，叩痛（+），松动Ⅱ度；颊侧牙龈处有0.5mm×0.5mm脓肿。X线片：根吸收达根长1/3，根分歧有低密度阴影。右下6根形成（Nolla分期8期），上方骨板消失。

右下5乳牙的治疗方法是（）

A.直接拔除术

B.根管治疗术

C.活髓切断术

D.去龋充填术

E.开髓引流后，根管治疗术