It was at the very beginning( )Mr.Smith made a decision ( )

It was at the very beginning( )Mr.Smith made a decision ( ) we should send for a doctor.A what that B that which C which that D that that

本题应该选择 D.that / that
解析
一、第一空处所以用 that 是因为It was at the very beginning that Mr.Smith made a decision 是强调句.
句中强调是时间状语 at the very beginning
如果不强调则为：Mr.Smith made a decision at the very beginning
强调的意思是“斯密斯先生正是在一开始就做出一个决定”,不强调则为“斯密斯先生一开始做出了一个决定”.
强调句的结构为 —— 先行词 it + be +被强调成分 + that + 句子的其他部分
用这个格式还可以强调其他成分,如：
--强调主语 Mr.Smith——
It was Mr.Smith that/who made a decision at the very beginning.（强调人还可以用 who 取代 that）.——正是斯密斯先生在一开始就做出了一个决定.
--强调宾语 a decision ——
It was a decision that Mr.Smith made at the very beginning.斯密斯先生在一开始就做出的正是一个决定.
二、 第二空处 that 是用来引导同位语从句的,即Mr.Smith made a decision (that) we should send for a doctor.
1、同位语从句的作用对前面的抽象名词做出具体解释,本句中 that we should send for a doctor（我们应该请一个医生）就是decision 的同位语从句,这里用来说明这个决定的具体内容.
同位语从句相当于汉语的“即”、“也就是说”.比如本句就可以直译为——
* 斯密斯先生做了一个决定,即我们应该请一个医生
* 斯密斯先生做了一个决定,也就是说我们应该请一个医生
* 斯密斯先生做了一个决定,这个决定就是我们应该请一个医生
也可以用 i.e.、that is to say 或者 namely 表示——
* Mr.Smith made a decision ,(that is to say) we should send for a doctor.
* Mr.Smith made a decision ,(namely) we should send for a doctor.
* Mr.Smith made a decision (i.e.) we should send for a doctor.
2、同位语从句的特点有三：
a）先行词是抽象名词,如 decision,news,fact,idea 等；
b）从句的句子结构完整,如本句中谓语动词 should send for前面有主语we,后面有宾语a doctor,不缺少成分.
c）关联词 that 不像定语从句那样充当句子成分,起到的只是表示这是一个名词性从句的提示作用.关联词 that 在正式英语中不能省略.