—How beautiful your hometown is!

—Yes, a number of trees _______ around Dazhou every year. Our environment is getting better and better.[ ]

A. are plant

B. are planted

C. are planting

D. were planted

（）具有机械强度高，化学稳定性好、廉价、取材便利等优点。不足之处是由于反冲洗的水力筛分作用，反冲洗后滤料层为小颗粒在上，大颗粒在下，其上小下大的孔隙分布不利于过滤，上部孔隙小易使滤层堵塞，下部孔隙大易使颗粒穿透。

A、石英砂滤料

B、反冲洗滤料

C、无烟煤石英砂双层滤料

D、均质滤料