Normal form of relation R

Question

It is given that for a relation R all the attributes of the relation appear in some of the candidate keys. It is also given that there is only one compound candidate key which exists for a relation, all the other candidate keys are simple candidate keys. What can be said about the normal form of R?

In my opinion, since all the attributes are part of some candidate attribute, it is surely in 3NF. But I am not sure if we can extend our reasoning to claim that R in also in BCNF(as claimed by the solution).

Can anyone please help me out why R can be said to be in BCNF normal form?

Thanks in advance!