Hi Sally,
Having checked the exercise, both célèbre and fameux are accepted as alternatives in the examples you quote but it is an interesting question as they can be used very differently:
Célèbre is only used positively to mean ‘well-know around the world’ and you would always use it for an actor for instance.
Fameux/fameuse can be used in a pejorative way to mean infamous so it is used with much care. It can also refer to the quality or reputation of someone or something.
E.G.
Ce restaurant célèbre n’est pas fameux = This famous restaurant is not first rate
In other words you don’t rate the food...
Hope this helps!
