How is it possible that queues occur even when the utilization is lower than $100\%?$

Group of answer choices

Variability causes the system to sometimes sit idle $($but this time cannot be stored$)$ and be really busy at other times $($leading to queues$)$

Because employees are lazy and wait for a queue to occur before they start working

The system always operates at maximum capacity.

Arrival and service times are constant and predictable.