A Manager Has Noticed That Her Workers Are More Likely

A manager has noticed that her workers are more likely to be in negative moods between 3 pm and 4:30 pm. She believes that employees will make valuable contributions to staff meetings when they are in positive moods, and that staff meetings cannot be successful unless the employees make valuable contributions. For these reasons, she has moved her daily staff meeting from 3 pm to 10 am.
If the manager's statements are all true, which of the following must also be true?
If the employees make valuable contributions during a staff meeting, then that meeting will be successful.
If the employees are in positive moods during a staff meeting, then that meeting will be successful.
If the employees did not make valuable contributions during a staff meeting, then they must have been in negative moods.
If the employees do not make valuable contributions during a staff meeting, then that staff meeting cannot be successful.
If the employees are not in positive moods during a staff meeting, then they will not make valuable contributions during that staff meeting.