Jack and Roberto were assigned to guard a tower. Each was to watch for 5 hours, then rest 5 hours while the other watched. If Roberto began his first watch at 6:00 p.m., at what time will he begin his third watch?

a) 10:00 p.m.
b) 11:00 p.m.
c) 12:00 a.m.
d) 1:00 a.m.