FD's={A->C,B->D}
$AB^{+}$ ={A,B,C,D}
So Candidate key is AB so A and B are prime attributes and C and D are non-prime attributes.
here in given FD
A->C
B->D
which means part of Candidate key determining non-key attribute or we can say partial attribute of key determining non key so it is not in 2NF hence not in 3 NF also.