An inventory system answers two key questions: when to order and how much to order. Which of the following statements correctly explains how a Q$-$system or a Psystem addresses these questions?

Under a P$-$system, an order is placed when on$-$hand inventory reaches the reorder point $R.$

Under a P$-$system, an order is placed to bring the inventory position up to the target level T every P time periods.

Under a Q$-$system, a fixed quantity is ordered every P time period.

Under a Q$-$system, there is a risk of stock$-$out during the protection interval.

Under a Q$-$system, an order is placed to bring the inventory position up to the target level T when the on$-$hand inventory reaches the reorder point R$.$