How does ibsen use dramatic irony to create suspense in this passage?

a)rank says that he thought nora would be dressed up, while helmer says that he did not expect her to be.
b)nora tells helmer that there are no letters in the box, while the audience knows that there is a letter from krogstad.
c)nora says that no one can see her in her costume until the party, where she will dance the tarantella.
d)helmer and rank promise to nora practice dancing, because she has not been practicing all day.

the answer it b