袋底位于牙槽嵴冠方的牙周袋称为
A．龈袋 B．骨上袋 C．骨下袋 D．复合袋 E．复杂袋
参考答案：B
Although you have lots of difficulties, you must _______your work.
A．carry on with
B．carry out with
C．carry away on
D．carry over
NULL是指
A．0
B．空格
C．未知的值或无任何值
D．空字符串
