因为grass，意思是草、草地，是不可数名词，例如：

1、Flatten the surface of the grass with a roller.用碾轧机把草轧平。

2、The plant resembles grass in appearance.这种植物的外形像草。

3、You're not supposed to walk on the grass.不准践踏草地。

4、The grass had withered in the warm sun.这些草在温暖的阳光下枯死了。