他在二十一岁生日时收到了一份特别的礼物。

He got a special gift for his ___twenty-first birthday.

二十一世纪见证了全球互联网的普及。

The ___twenty-first century has witnessed the widespread adoption of the internet globally.

他们计划在结婚二十一周年的那天去欧洲旅行。

They planned a trip to Europe for their ___twenty-first anniversary.