theory of optimum population for a closed economy

例句：

1. 1. The theory of gravity explains why objects fall to the ground.
   万有引力理论解释了物体为何会落向地面。
2. 2. This theory has been proven by numerous experiments.
   这一理论已被许多实验证实。
3. 3.He proposed a new theory on how atoms interact with each other.
   他提出了一个关于原子如何相互作用的新理论。