Is the relation in 3NF?

R(ABCD) with FD's

AB -> CD BC -> D

I found this question in a book and it says the relation is in 2nf but not in 3nf.

I cant find any trivial dependencies and also no non prime attribute is functionally dependent on another non prime attribute then how come the relation is not in 3nf?

Should i consider BC as a non prime attribute too? Please help me understand the concept of 3NF