牙骨质龋属于
A．慢性龋
B．浅龋
C．中龋
D．深龋
E．继发龋
参考答案：B
数据链路层分为哪两个子层？（）
A.逻辑链路控制子层
B.接入层
C.介质访问控制子层
D.核心层
John thought it wouldn’t be long _____ he was ready for his new job.
A．since
B．when
C．before
D．after
