[求助]请教两道LZM语法题

请教两道语法题:-

1. People who inherit the sickle cell anemia gene from only one parent seem to be resistant to malaria, an evolutionary advantage that may explain why a genetic condition so debilitating to many individuals has survived ni the human population.

(A) seem to be resistant to malaria

(C) seem to be resistant to malaria and have

为什么是选A, 而不是选C呢? People have evolutionary advantage 也很通啊.

2. 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.

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

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

为什么选D, 而不是选B呢? "20% of the leaf and small stem material" 难道是单数吗?

an evolutionary advantage 是指前面的提及的情况，也就是resistant to malaria，意思明确;

而C将have与seem并列，意思是这些inherit the sickle cell anemia gene from only one parent 的人即seem to be resistant to malaria又have an evolutionary advantage，而这个advantage是什么？并没有解释；

所以选A.

那 "has survived in the human population" 的主语是什么呢?

还有谁可以解答第二个问题啊?

1, have may parallel with seem or to be; and because the appositive turns to coordination part, it becomes vague what is the advantage.

2, The subject of the sentence calls for singular verb.

应该是a genetic condition