请问一白勇题！

In the most common procedure for harvesting forage crops such as alfalfa,as much as 20percent of the leaf and small-stem material,which is the most nutritious of all the parts of the plant,shattered and fell to the ground.

D the most nutritious parts of the plant,shatters and falls

as much as .......后面怎么是修饰material？怎么用单数谓语动词呢

as much as 應是修飾 20 percent

那为什么主语不是leaf and material呢？

是material做主语啊