What are known patterns for the adaptation of interfaces?

I have got a sample question from CPSA exams. it is like that

What are known patterns for the adaptation of interfaces? (choose two answers) A. Bridge B. Tower C. Observer D. Facade E. Wall

The answer is Bridge and Observer. I can understand why bridge is. But observer? i have absolutely no clue. Could somebody give me some the reason why observer is adaptation of interfaces. in this case i would choose Facade and Bridge. however Facade is not the correct answer.