The most important thing we should pay attention to is the first thing I have said.A. w

问题补充：

The most important thing we should pay attention to is the first thing I have said.

A. which; that

B. that; which

C. that; that

D. which; which

答案：

A. which; that

B. that; which

(答案→)C. that; that

D. which; which

解析：考查定语从句：句意：我们应该注意的最重要的事情是我要说的第一件事。两个句子都是定语从句先行词分别是The most important thing ，the first thing 当先行词被最高级或序数词修饰的时候，定语从句用that引导，选C。