釉质覆盖于牙冠表面，最厚处位于
A．牙尖及切缘处
B．牙颈
C．牙髓
D．牙龈
E．牙本质
参考答案：A
The speech which he made ______ the football match bored a lot of fans to death.
A．being concerned
B．be concerned
C．concerned
D．concerning
You have to be six years old before you _____ to go to school. [ ]
A. will allow
B. are allowed
C. have allowed
