患者女性，25岁，上前牙光敏树脂贴面半年余，近1个月觉刷牙牙龈出血，龈 * * 呈球状增生，质地松软。

最有可能的诊断（）

A.青春期龈炎

B.肥大性龈炎

C.药物性牙龈增生

D.边缘性龈炎

E.青少年牙周炎

How did your brother make such a stupid mistake ______ he had a such a good knowledge of the

steps and cautions over operating the machine.[      ] 

A.  when  

B.  because  

C.  while  

D.  that