Sigma.
[医] 千分之一秒
🗣️ 情景例句
You're welcome.
不用谢
How long have you been studying English?
你学英语多久了？
From these he can go on to appreciate more complex forms or combinations of several forms.
从这些形体出发，观赏者可进一步观察更为复杂的形体或若干形体的组合。