10
(A)racial
(B)crucial
(C)spatial
(D)cultural

crucial (a.) extremely important 

後半句: 但是最重要的原因是 大腦的力量 

What is particularly interesting about our species（動植物的）種，物種?

For a start, we walk 7 on our hind-legs后腿 at all times, which is an extremely unusual way of getting around for a mammal哺乳動物.

There are also several unusual 8 about our head, not least of which is the very large brain it contains.

Our forelimbs前肢, being freed from helping us to get about, possess a very high degree of 9 skill.

Part of this skill lies in the anatomical解剖 structure of the hands, but the 10 element部件 is, of course, the power of the brain.

10 
(A)racial翻譯：種族的；種族間的；種族關係的, 人種的，種族的；作為人種（或種族、民族）
(B)crucial翻譯：至關重要的；關鍵的；決定性的。
(C)spatial翻譯：空間的。
(D)cultural翻譯：生活方式, 與文化有關的，文化的, 文化的, 文化的，文藝的。

Crucial 決定性的、重要的