What Is the Correct Ruling in the Following Case? an Employer

What is the correct ruling in the following case? An employer announced that it was going to close a union facility, and entered into negotiations with the union. Six drivers learned of an upcoming meeting, met over coffee to formulate their questions, and went to the site of the meeting. A union official told them to return to work, but the drivers insisted, and eventually were able to introduce themselves to the management representatives. They returned to work after having been gone for 3 hours, but were fired for being absent without authorization. Was this termination legal under the NLRA? What should the NLRB decide?:

A) for the employer, since the employees essentially walked off the job during working hours without authorization, which is not a protected concerted activity
B) for the employer, since its representatives had met with the employees, so they had complied with their obligations
C) for the drivers, since they had a right under the NLRA to engage in protected concerted activity
D) for the drivers, because although they were able to introduce themselves to the management representatives, no actual discussion or negotiation took place