与固定义齿自洁作用有关的形态是
A．牙冠形态B．牙根形态C．桥体面形态D．连接体形态E．桥体龈面形态
参考答案：E
The teacher often tells us_____our classroom_____.[ ]
A.to keep; to clean
B.keeping; cleaning
C. to keep; clean
D.keep; clean
机车的蛇行运动是怎样形成的？
