An inventory system answers two important questions: when to order and how much to order. Which one of the following statements correctly explains how a $Q$ model or a P model answers these questions?

Under a P model, a fixed quantity is ordered when the inventory position reaches the reorder point $R.$

Under a P model, a variable quantity is ordered to replenish the inventory when the review period $(T)$ arrives.

Under a Q model, a variable quantity is ordered when the inventory position reaches the reorder point $R.$

Under a Q model, a fixed quantity is ordered every T time periods.