How men first learned to invent words is unknown; in other words, the origin of language is a mystery. All we really know is that men, unlike animals, somehow invented certain sounds to express though
When I was six, Dad brought home a dog one day, who was called “Brownie”. My brothers and I all loved Brownie and did different things with her. One of us would walk her, another would feed her,
(x)最分散，其次是
(x)，而
(x)相对最集中，所以σ1＞σ2＞σ3.