答案：D

题目分析：句意：那是我们的信念医疗保健的进步将促进一个更强大,更繁荣的经济。这里是that引导的是主语从句，它在从句中不作成分，it是形式主语，代替真正的that引导的主语从句。故选D。

点评：本题难度适中。代词是高中阶段要求掌握的语法项目，尤其是it的用法多，并且还是高考常考的知识点，需要考生在平时反复的练习中巩固掌握它的用法。

即学即练：____ was known to us all that Johnson had broken his promise____ he would give each of us a small gift.

A. As; which       B. What; that         C. It; that     D. It; which

解析：C。It为形式主语，代替真正的主语从句that Johnson had broken his promise ...；that引导同位语从句，解释promise的具体内容。