单选：先天缺额牙最多发生在（）。A．上颌侧切牙B．上颌第二前磨牙C．下颌第二前磨牙D．上下颌第三磨牙

简述闭口肌群的组成及功能。

If you decided to learn a new language, you would have to devote all your eff...

He has taught English in this university ever since he moved to this city.

In no other country in the world can you find such plants as this one.

This place has plentiful material resources.