The reason often given for the low regard in which smell is held is that, in comparison with its importance among animals, the human ...?

原文是:
The reason often given for the low regard in which smell is held is that, in comparison with its importance among animals, the human sense OF smell is feeble and undeveloped.
嗅覺之所以被輕忽，常給予理由是: 與其在動物間至關重要相比，人類的嗅覺遲鈍而且未充分成長。

這是一個復雜的句子，一步步猜解構:
1.) The reason often given for (the low regard in which smell is held) is that,
括號內先不看，或用代號 X 替代
The reason often given for X is that,... 常被引用說明 X 的理由是...
= The reason (which is often given) for X is that... which is 可省略

2.) the low regard in which smell is held
英文說極為尊敬某人 hold sb. in high esteem，說輕蔑，看輕某人或某事 hold sb. or sth. in low regrad
所以原句也可以寫成
The reason that smell is held in low regard is that... 但沒有原句來的有深度


a.) 這句話的given是pp嗎? ==> 是的
b.) in which的用法是因為後面的held嗎? ==> 來自於hold in high regard 變化運用
c.) that後面的句子是在說明that嗎? The reason (for whatever) is that ... ==> that 後面是the reason 的真正原因

(1)The reason often given------pp of give--->has often been given=present perfect in grammar passive in technical term---Re:-be--->been--->an example of ellipsis when the leaving out of a word from sentence when the meaning can be understood.
for the low regard in which "smell" is held----------adv.cl as S;in which=where=where to introduce a cl.in which something happens(="smell" is held)---->Which is used when a choice is to be made from a known set of things,eg:-smell,sight, hear,touch,taste;that one of the five senses which is special to the nose,which is noticed by means of the nose; in which=in one of the 5 senses which----in which case=if this is true for smell=if smell is the difference between " which is which"whichever&which are both determiner,pronoun.
(2)is----V
(3)that the human "sense if smell" is feeble&underdeveloped----------noun cl. as O
(4)in comparison with its importance among animals.---------adj. phr. to (3)