Question

the productivity of a person at work (on a scale of 0-10) is modeled by a cosine function: 5cos (pie/2t) +5, where t is in hours. If the person starts work at t=0, being 8:00 am, at what times is the worker the least productive?
a) 12pm
B) 10 am and 2pm
C) 11 am and 3 pm
D) 10 am,12pm and 2 pm