For 3rd point,
B problem is NP and A is NPC problem as given in question.
We know all NP problems are reducible to NPC problems.
So B --> A
Now if there exist a Polynomial time Algo for A ...hence A is decidable.
Then from B-->A we say B cannot harder than A and A is decidable ==> Hence B is also decidable i.e there exist a P time algorithm for it.