is our belief that improvements in health care will lead to a stronger more prosperous

问题补充：

is our belief that improvements in health care will lead to a stronger, more prosperous economy.

A. As

B. That

C. This

D. It

答案：

A. As

B. That

C. This

(答案→)D. It

解析：通过分析句子结构，可知这个句子中包含一个主语从句，且主语从句位于句末，故用形式主语It。