Below the topsoil of the earth is the subsoil, _____ layer that contains stones mixed with a small amount of _____ organic matter.

A．a; the

B．the; the

C．不填; 不填

D．a; 不填

（）对节约建设投资、降低生产成本、提高投资效益具有重要意义。

A.节能

B.节材

C.节水

D.节地