index of import and export

例句：

1. 1. The index of the stock market has been rising steadily for the past few months.
   过去几个月，股市的指数一直在稳步上升。
2. 2. The index of the economy is closely watched by economists around the world.
   经济指数受到全世界经济学家的密切关注。
3. 3. The index of happiness is a measure of how people feel about their lives.
   幸福指数是一个衡量人们对自己生活感觉的指标。