Final answer:
Empty expansion bays in a computer case need to be covered with slot covers to preserve the intended airflow and avoid dust accumulation inside the case.
Step-by-step explanation:
Empty expansion bays must be covered by a slot cover to maintain proper airflow inside the case. When a bay is left open, it can disrupt the planned flow of air, which is designed to cool the internal components effectively. Using a slot cover ensures that air is directed over the components that need cooling, like the CPU and GPU, and prevents dust from settling inside the case.