The teacher spoke loudly ____ the students could hear him clearly. A. so B.that C.so that

我想这道题后面应该是一个目的状语从句吧，有的资料上说that，so that都可以引导目的状语从句，还有的说so也可以，那我想知道在引导目的状语从句上，so， that 和so that的区别。如能解决我的疑惑将非常感谢！
我问的是为什么不选 that ？ that 也可以引导目的状语从句啊。

so因此，所以，表结果
that代词1. that 用作指示代词（复数形式是those），其指代意义同形容词用法，同时它还可以用作定语从句中的先行词；还可为了避免重复，代替前述名词。
That is what he told me.
What is that (which) you have got in your hand?
The price of rice is higher than that of flour.
2. that 用作关系代词，引导限制性定语从句。先行词可人可物，用法相当于who或which。（但是在下列情况下多用that：先行词既有人又有物时；先行词有形容词最高级、序数词、不定代词、very，only等修饰时；先行词是不定代词时）
He talked about the teachers and schools that he had visited.
I think it one of the most wonderful films that the film company has ever produced.
She has little information that is useful for our research.
Is there anything that I can do for you?
请注意，that 在定语从句中作宾语时通常可省略。
The books (that) I sent you will help you in your studies.
so that 如此。。。以至于。。。这道题选C。这位老师说的如此大声以至于学生能够清楚地听到他说话

so,,,that 如此，以至于。so that 以便为了。 意思不一样。

c 试题分析：句意：，为了让学生听清楚老师大声讲话。so that 以便，为了，引导目的状语从句； as a result 结果； in order to为了，后跟动词原形；anyway无论如何。所以选c。

1.单独的that不能引导目的状语从句。必须有so that 或者 in order that等一起用。 2 so。。。that。。。如此。。。以至于。。。 so that 以便，为了。。。