20percent of+ 不可数名词1 and 不可数名词2+ be

这边的谓语应该是单数还是复数啊~

In my opinion:

1. X percent of n.: n. is the decisive factor in the agreement of subject and predicate.

   a) n. singular      ->      v. singular

   b) n. plural        ->      v. plural

2. n1. and n2.: 

   a) whatever none or only one  or both of n1. and n2. are singular, as long as the two words present different things     ->     v. plural

   b) both n1. and n2. are in singular forms and the two words present only one combined thing      ->     v. singular 

     eg: Bread and butter is my favorite.   (Bread and butter is one kind of combined food.)

To be discussed.

Thanks for reply~

The question is actually:

大全416.        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 fall

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

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

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

答案是D