Form 15- Q19

[puresillylily]

As vital as their igloos, which permit the Innuits to live in reasonable comfort in an icy land, and just as ingenious, are their winter clothing, so perfect in design and material that the Innuits are made nearly impervious against any arctic weather.

(B) are the winter clothing of the Innuits, so perfect in design and material that they are made nearly impervious to
(C) is their winter clothing, so perfect in design and material that it makes them nearly impervious to
(D) is the winter clothing of the Innuits, so perfect in design and material as to make them nearly impervious against
(E) has been the winter clothing of the Innuits, so perfect in design and material as to be made nearly impervious to


Key to the answer: "clothing" is a singular noun, and requires a singular verb. Thus, options (A) and (B) are out.
But, my question is - how to analyze the structure of the sentence?

As vital as their igloos(,which permit the Innuits to live in reasonable comfort in an icy land, and just as ingenious,) is their winter clothing, so perfect in design and material that it makes them impervious to any arctic weather.

這個 "So...that" 的 "so" 是什麼詞??? 可以給其他例子嗎?


Thank you!