____ the people have become masters of their country _____ science can really serve the people.   [ ]

A. It is only then; that    

B. It was that; when    

C. It is only when; that  

D. It was when; then

“拉弗曲线”是美国经济学家拉弗用来描绘税收与税率关系的曲线。下图是根据“拉弗曲线”而复制的另一曲线。这条曲线告诉我们（）

A.税率的高低对财政收入的影响是基础性的

B.税收是国家筹集财政收入最普遍的形式

C.财政收入的增减与税率的高低成正比

D.合理的税率有利于增加国家财政收入