why BC is not a candidate key and others are please provide explanation

Consider the following database table having A, B, C and D as its four attributes and four possible candidate keys (I, II, III and IV) for this table:

A B C D
a1 b1 c1 d1
a2 b3 c3 d1
a1 b2 c1 d2

I:(B)

II: ((B,C)

III: (A,D)

IV: (C,D)

If different symbols stand for different values in the table (e.g. d1d1 is definitely not equal to d2d2) then which of hte above could not be the candidate key for the database table?

 

  1. I nad III only
  2. III and IV only
  3. II only
  4. I only
2Comments
shivani @shivani1234
6 Oct 2017 11:20 am

none of the above 
as all the given options are possible candidate keys 

Pun m @punm
8 Oct 2017 02:55 pm

i also got the same but the answer key of ugc net gives BC is not candidate key  then i came to conclusion that c is not derive uniqe thats why BC is not candidate key . AM I RIGHT