在人群中观察在增加或消除某因素后某病发病率的变化，该研究属于哪种流行病学研究方法

A. 补中益气汤
B. 四君子汤
C. 六君子汤
D. 归脾汤
E. 以上都不是

A. an increase in the supply of money and a decrease in the velocity of money
B. a decrease in the supply of money and an increase in the velocity of money
C. the inverse relationship between the supply of money and nominal GDP
D. deficit financing which increases interest rates and reduces investment