请教刘振民语法里面一道关于主谓一致的题目！

 11-38

In the most common procedure for harvesting forage crops such as alfalfa, as much as 20 percent 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.

(A) which is the most nutritious of all the parts of the plant, shattered and fell

(B) the most nutritious of all parts of the plant, shatter and fell

(C) the parts of plant which were most nutritious, will shatter and fall

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

(E) parts of the plant which are the most nutritious, have shattered and falled

答案：D

请问下这道题书上说根据主谓一致选D，那就是说as much as 20 percent of the leaf and small stem material是一个单数概念。但是...percent of...的单复数不是由of后面的部分决定的吗？这里面of后面是and结构连接的两个名词，那样不是应该是复数概念吗？请高手指教！

谢谢指教！