With the country’s population increasing to 1.3 billion, many of China’s rivers, including the legendary Yellow River,       .

A．is drying up

B．will be drying up

C．had dried up

D．are drying up

（）是指颗粒尺寸由大到小连续分级，其中每一级石子都占适当的比列。

A.最大粒径

B.间断级配

C.连续级配

D.单粒级