按照流动的参照物划分，社会流动可分为（）

A.水平流动和垂直流动 

B.代内流动和代际流动 

C.结构性流动和自由流动 

D.向下流动和向上流动

It is said that the old coins       under the earth for about 100 years by the time they were discovered.

A．had buried

B．had been buried

C．buried

D．were buried