成人牙齿过早松动脱落的主要原因是
A．肾阳虚衰
B．肾阴亏虚
C．肾气不固
D．肾精不足
E．肾不纳气
参考答案：D
成岩阶段划分？主要成岩作用类型及其特征？
—Do you know our town?
—No, this is the first time I _______ here. [ ]
A. was
B. have been
C. came
D. am coming
