一般说来，产量构成因素很难同步增长，往往彼此之间存在着（）关系。
参考答案：负相关
— Americans spend more on health care than most other people .
— You’re right . But a new study shows that life ___ in the United States is falling . [ ]
A. development
B. position
C. experience
D. expectancy
得神
