Understanding 3NF: plain English please

Question

I am working through an example problem in which we are trying to identify which of the following relations is in Third Normal Form (3NF). Here are the relations we are given:

Answer 1

In simple english, here are the 3 Normal Forms:

1NF: The existence of "the key" ensures that the table is in 1NF(Key must be there).

2NF:It is required that "each" non-key attributes be dependent on "the whole key" to ensure 2NF.

3NF: further requiring that "each" non-key attributes be dependent on "nothing but the key" ensures 3NF.

Now, for this:

R1(ABCD) ACD -> B   AC -> D   D -> C   AC -> B

Look at these ACD -> B and AC -> B: clearly violates the 2NF condition. Forget 3NF, this relation is not even in 2NF. "the whole key"-->concept does not hold.

I think, you have proved the same using SET.