The Main Goal of a Pulse-Demand Oxygen-Delivery System Is Which

The main goal of a pulse-demand oxygen-delivery system is which of the following?
1) To provide oxygen only during expiration
2) To provide oxygen only during inspiration
3) To deliver oxygen to the patient only after a sufficient inspiratory effort is made and the demand valve opens
4) To deliver oxygen at a preset flow rate and close during inspiration to conserve oxygen

A) 1 and 3
B) 2 and 4
C) 2 and 3
D) 1 and 4