关于成牙本质细胞，正确的是

A．是具有增殖潜能的细胞

B．只形成牙本质基质，不参与牙本质矿化

C．细胞体位于多细胞层，胞质突起位于无细胞层

D．是具有极性排列的上皮细胞

E．能合成牙本质磷蛋白

There is a new problem involved in the popularity of private cars _____ road conditions need _____.

A．that, to be improved

B．which, to be improved

C．where, improving

D．when, improving