在成牙本质细胞和矿化牙本质之间形成的一层未钙化的牙本质
A．前期牙本质B．继发牙本质C．球间牙本质D．透明牙本质E．髓周牙本质
参考答案：A
调车信号因故不能开放或机车车辆压原排调车进路须中途原路退回时怎样行车？
I_____the problem _____in my mind for a whole week before I did anything about it.
A．switched; on
B．kicked; up
C．turned; over
D．took; on
