解释一道关于so...that 和Such...that的题目 具体区别？详解。

Question

Their helmets covered their faces _ that it was impossible to recognize evem one's best friends,
A.to so an extent B.to as extent so C.to such an extent D.to a such extent
正确答案D

Answer 1

to an extent 达到····程度
so...that 和Such...that区别主要是 so通常后接形容词
such后通常接名词
这里的extent是名词 所以用such

下面是参考语法
比较so和 such

其规律由so与such的不同词性决定。such 是形容词，修饰名词或名词词组，so是副词，只能修饰形容词或副
词。so 还可与表示数量的形容词many，few，much， little连用，形成固定搭配。

so + adj. such + a(n) + n.
so + adj. + a(n) + n. such + n. (pl.)
so + adj. + n. (pl.) such +n. (pl.)
so + adj. + n. [不可数] such +n. [不可数]

so foolish such a fool
so nice a flower such a nice flower
so many/ few flowers such nice flowers
so much/little money. such rapid progress
so many people such a lot of people

so many 已成固定搭配，a lot of 虽相当于 many，但 a lot of 为名词性的，只能用such搭配。
so…that与such…that之间的转换既为 so与such之间的转换。

Answer 2

首先，这个句子的关键不是so...that和such...that的区别，而是extent这个词。extent这个词通常用于to+ some/what/such an/ a certain+ extent短语中，表示达到某种程度。在这个句子中，即为to such an extent的变体。
从另外一个角度看，so后面应该接形容词，不接名词；而such后面可接形容词，也可接名词。
如果非要在这个句子中用so...that, 就应该这么说：Their helmets covered their faces so heavily that it was impossible to recognize even one's best friends.