answer: #Person1# feels terrible knowing one-third of the students left during the class, then #Person2# says it's because the lecture isn't today.
documents: #Person1#: Did a lot of people attend the lecture?
  #Person2#: Well, yes and no.
  #Person1#: What do you mean?
  #Person2#: You see, the classroom was quite full at the beginning. Almost all the seats were occupied. But after the short rest, almost one-third of the audience left.
  #Person1#: Terrible! That was a very rude thing. I should have been there. I had told the students to behave themselves. I do hope Professor Black was not angry.
  #Person2#: What do you mean? He wasn't even there.
  #Person1#: What? Professor Black did not come?
  #Person2#: No, his lecture is Thursday next week, not this Thursday.
  #Person1#: Oh, yes, of course. How silly of me!