In Three of the Following Situations, Ignoring a Student's Misbehaviour

In three of the following situations, ignoring a student's misbehaviour might be the best approach to take.In which situation would ignoring not be advisable?

A) Anita has whispered a juicy (and apparently hilarious) piece of gossip to her friend, who in turn is passing it along to two others.
B) Rick breaks a beaker and seems very remorseful that he has done so; he vows to be more careful in the future.
C) Michelle is usually very attentive in class, but today when the teacher asks her a question, it is clear that she has not been listening.
D) An entire class seems to be on a "sugar high" the day after Halloween.