a.

1. 出名的，眾所周知的[（+for/as）]
   He is one of the world's best-known opera singers.

   他是世界上最有名的歌劇演員之一。

2. 熟知的
   She sang an old well-known song.

   她唱了一首大家熟悉的老歌。

   For reasons well-known to all, I had to cancel the trip.

   鑑於眾所周知的緣故，我不得不取消這次旅行。