半元音（无擦通音）
参考答案：
发音的持阻阶段口腔通道只形成很小的阻碍，气流通过时，只产生极轻微的摩擦，就产生半元音。
The National Education Department says again school safety has set off alarm bells with frequent serious
accidents _____ students got injured or even killed. [ ]
A. that
B. when
C. in which
D. from where
牙龈卟啉单胞菌与下列哪项关系最密切（）
A.青少年牙周炎
B.成人牙周炎
C.快速进展型牙周炎
D.难治性牙周炎
E.ANUG
