Tom wants to be a singer and he_____. singing lessons to do it.    [ ]

A. took        

B. has taken    

C. is taking    

D. was taking.

一般认为根管充填的时机是

A．髓腔已完全清理、扩大和成形

B．无自发疼、叩诊无反应

C．窦道口封闭、根尖区牙龈无红肿，无压痛

D．无腐败臭味、根管内无脓和血

E．以上全对