Until the 18th century mankind's output had been restricted by the amount of physical force that humans (and domesticated animals) could exert and by the amount of wood that people could chop down.
直至18世纪前，人类的产出仍受制于人(及家畜)的体力以及砍伐树木的数量。
2
Output operand expressions must be lvalues. The input operands are not restricted like this. They may be expressions.
输出操作的表达式必须是数值，输入操作没有这个限制，他可能是表达式。
3
In this way, the output of PSF is restricted, the quality of product is decreased and unit energy consumption of pellet is increased.