按桥体龈端的形态分类不包括
A．盖嵴式桥体
B．改良盖嵴式桥体
C．船底式桥体
D．接触式桥体
E．悬空式桥体
参考答案：D
I'm sorry that I can not accept your ________ to attend your birthday party, Linda, because I'm
________ to answer all the customers' letters tonight by my boss. [ ]
A. requirement; required
B. demand; requested
C. request; asked
D. request; required
重为6N的木块放在斜面上，画出它所受重力图示和支持力的示意图。
