"在二十世纪90年代" 是 in the 1990s.二十世纪翻成英语变成了19那么"在二十一世纪" 是 in the 21st century 还是 in the 20th century?

In the 21 first century is the right one,
In the 1990s referres the year between1990-1999
The 21st century referres to 2001-2100
It is hard to explain ,just remember it

in the 21th century
在年份中**世纪要在本世纪后－1
但直接读的时候不变 21世纪还是21th century